First of all I'm very sorry to hear about your partner's injury. I hope you're making full use of all the various types of support you have available. I have a Motability car too, but I'm considering going back to private ownership next time around (no fault of Motability). What follows is a little long compared to most posts, but I make no apology as it's (1) relevant and (2) is genuine advice based on real personal experience. I hope it helps you.
Because of a Daily Mail style outcry a few years ago, the scheme has now been capped to cars of up to £25,000 before VAT (price to Motability not retail). This sounds like a lot, but these days with ever rising prices it is actually somewhat of a restrictive budget in many cases. For example even my Skoda (Superb 2.0 TSI) retails at £30-odd grand, but just fell within the scheme. I therefore had a choice of getting the DSG (which we need for health reasons) and not being able to get 4x4 as well, or else dropping down to accept a diesel and/or manual gearbox if we wanted/needed 4x4. The combination of petrol, plus DSG plus 4x4 wasn't on the scheme, because it scraped above the cap. This happens with quite a few cars of various marques, and in many cases not for such 'high end' reasons, so it's something to bear in mind. For example - just an example - you could find you can get a higher '3' trim Kia with a diesel/manual but only a '2' in auto or petrol (or vice versa). Bear in mind that year on year car prices rise - often by a thousand or two at a time - but the cap has remained in place without adjustment for some years now.
To pre-empt some of the more stupid replies: Yes, boo hoo the 'free car' isn't fully loaded (in all cases) if you go for the spec you want... But back in the real world, you're
spending a cash in hand benefit on a lease, when you could just as easily have spent it on hookers and booze if you wanted to. Or, more pointedly, you could spend it instead on a privately owned car of the spec you actually want/need.
So the cap IS a consideration for disabled customers looking to lease, but it's also stupid and based on the prejudice of healthy people who don't know their own luck. Why on earth should 'they' want to stop people spending their own cash benefits on a more expensive car, rather than being restricted to 'lesser' cars and then potentially wasting the extra cash on aforementioned booze and hookers instead? Madness, especially as a good percentage of DLA/PIP claimants also work full time jobs. Who should be dictating what they can spend their cash on?! So in summary, can you get what you want for the cash/on the scheme? It's worth looking.
There
are a few cars on the scheme that are fun to drive, or at least powerful/brisk/loaded (but not necessarily all of those things). It depends on your criteria and what you like/want/need. Motability let you search based on things like how 'green' a car is, or what size it is, but they never really mention 'fun' or 'sportiness'. It just attracts the aforementioned idiots who complain about disabled people still having a life.
So, you need to have a good nose about their list and it helps if you know what you're searching for. As I said we opted for the 2.0 TSI 220ps DSG this time around. It's certainly fast enough (0-60 is about 6 seconds measured, but VAG typically understate it at 6.9, because 'Skoda, not VW or Audi'), and it's huge and comfortable so it ticks the family and mobility needs requirement. It's not a sports saloon, and it's not 'fun' in some senses, but it does the job compared to the 7 seat diesel mum-mobile we'd have ended up in otherwise. Had I been less restricted (i.e. less children) then I'd have had a lot more possibilities even down to sporty two seaters!
As suggested above, with just the two of you (?) you can also have the luxury of broadening your scope to the medium/smaller cars in the 'catalogue', which broadens your options a lot. With two adults and soon to be three children, ours was a more limited search, especially as like you we had performance petrol on the list of requirements.
You don't mention mileage in your OP (that I saw, it's late), but I'd definitely suggest looking at petrols rather than just the diesels you're looking at on the scheme. No matter what people say, performance and diesel just don't really go together, especially not at the sub 25k end of the market and certainly not if you want actual 'driver enjoyment' and engagement rather than just outright 0-60 times - which admittedly some of the bigger multi-turbo diesels are good at now even if they are linear, isolated and transfer none of the sensation of acceleration to the cabin.
For example, just sticking with the Motability fleet for now, in the real world would you really get more driver enjoyment out of a 2.0 185ps Focus diesel (ST badge or not) compared to, say, the 2.0 240ps ecoboost Mondeo, or the 1.5 ecoboost Focus? Or like us the 2.0 TSI in a Superb, Octavia, or Seat of some description? Don't let ST/VRS badges sway you when it comes to diesels (just IMHO). I'd much rather take a 1.5 ecoboost Focus petrol and have a blast wringing its neck along the B roads than suffer a 2.0 diesel because it had an ST badge...
Also consider the newly released Vauxhall Astra. Yes it's a Vauxhall, but it's exceedingly well reviewed. All new design, smaller outside than ever, bigger inside than ever, actually well built, superbly comfy, 200KG lighter than before, better suspension and more connected steering... and it throws out 200BHP from a 1.6 turbo petrol that still returns 40mpg in the real world! It's on Motability's list, but you only see it if you know to look for it.
If fuel budgets really are tight and you do the big miles, then a 1.0 to 1.5 ecoboost or TSI engine will be just as efficient as most of the comparable diesels real world, and much more engaging. You'll also find plenty of space in the back of a Superb (or Mondeo) for a wheelchair... plus a week's shopping and a few suitcases!
For example even our 2.0 220hp turbo petrol is giving us almost identical MPG week on week compared to our previous 2.0 TD, and the 2.0 TD Mondeo courtesy car. There is literally 3mpg in it most weeks - barely a couple of quid a week for 100 extra HP and a proper, refined and fast engine.
With the greatest of respect though, if you're considering Motability versus spending £15k cash on your own car I'm suspecting that you'll not worry too much about a fiver a week difference at the pump if you go Motability and petrol...
The scheme has its quirks but it is reliable and hassle free. Your wife qualifies for
free disabled class VED regardless of being on the scheme or not (I hope you knew that and claimed it for the Cooper S), but on Motability you get a brand new car with the (free) tax renewed automatically every year, fully comp insurance, full breakdown cover and inclusive tyres, servicing and replacement parts like wipers/brakes etc. It is, as they like to say, hassle and worry free motoring.
They look after you if anything goes wrong, too. I recently had someone crash into our (two week old!) Superb while it was parked up. They drove away without leaving details and Motability and RSA - their insurer - were great. The RAC picked up the car within the hour and took it to the approved crash repairer, and I was given a nearly new Mondeo for the few weeks repairs took (parts delays at Skoda's end). The 'official' courtesy car was listed a Corsa or similar, but RSA/Motability pointed out unprompted that I was a tall bloke and my wife was pregnant so that wouldn't be suited to our needs. They had an automatic Mondeo delivered to us the next morning (it was late on at the time), and then collected it when our car was dropped off all fixed. No hassles, no worries, no problem.
The scheme certainly takes out a lot of the usual 'hassles' (albeit small ones) of car ownership. You never need to worry about servicing, sourcing insurance, MOTs, consumable parts, or breaking down. Everything is covered one way or the other and they do look after you. The downsides, small as they are, do need considering though as I said. Certainly 99% of people - even most petrol heads - would find something they'd be happy enough driving even with the scheme's modest restrictions.
But if you're particularly anal about your car choice, want full control over garage selection/parts used/engine and drivetrain spec or whatever, then yes you should consider buying your own car. If you do very much higher than average mileage you also probably will find the scheme doesn't suit, as Motability 'only' allow hirers an average of 20,000 miles a year (i.e. 60k over the three year lease) before mileage charges of (iirc) 5ppm are incurred. That's 385 miles a week, every single week for three years though, to be clear.
Also it's true that you 'lose' the money you paid, in the sense that like with any other car lease you have no asset at the end that you own. As you pointed out in the OP, though, you do have to carefully consider the lease cost
minus what you'd have paid for insurance, servicing, tyres, breakdown cover etc on a privately run car. The figure shrinks somewhat, especially once you factor in depreciation on a like-for-like private purchase. As you concluded, total cost of ownership boils down pretty close to the cost of the lease anyway. But of course if you buy a slightly older second hand car, the figures start to tip the balance in private ownership's favour. You're starting to compare apples to oranges when you do that, though.
Browse
Motability's search, get an idea of what you want and visit a lot of dealers. A lot of dealerships have excellent Motability trained sales staff (they are all trained by Motability before being allowed to sell Motability cars), but at the same time a lot of dealerships have very disinterested 'sales' men in the other sense of the word, who just want you in and out, have no interest in your needs, and just want an easy sale.
Push for extended test drives, as 10 minutes round the block will NOT show your wife if she'll be comfortable in the car long term. I was considering a BMW 320i last time around, and the BMW Motability sales guy literally only let me drive around the block (out, turn left, turn left, turn left, back into the car park). They were genuinely shocked when they rang me after I didn't place an order. Meanwhile Skoda took us out for over an hour - without us asking - across town, city, countryside and motorway so we could 'fully assess the car for our needs'.
So push for long drives and even repeat tests as and where needed, and make sure that you can get into, out of, and around the car easily. Are the cubbies/glovebox/cupholders/controls easy to access and use? Are the doors going to open wide enough, or too wide, and are the seats and seatbelts suitably accommodating? Is the seating position and H point high (or low) enough, and does it make getting into and out of the car on a bad day impossible or just more difficult than it would need to be? Does it have the kit (sensors, nav, whatever) and visibility you and/or your partner will need to use the car safely and comfortably? Is the load space wide, easy to access and at a suitable level without becoming wearing over time? Will you have room for extra passengers, luggage or shopping after the wheelchair is loaded? Do the rear seats split and fold, and is it in a convenient manner?
Loads of things to consider that 'normal' people don't even have to give a second though. Don't be rushed, and definitely view and test as many cars, engines and marques as you possibly can before committing. Anyone who pushes you or isn't overly helpful should be reported to Motability - just walk away and go somewhere else. There are literally thousands of other dealers who will be happy to help you into the same car without the BS. Voice of experience talking.
Sorry this went on but it's not a straightforward question, and there are a lot of things to consider. I just wanted to cover some of them to help you out and hopefully put your minds at rest. If you need any help, have other questions or just need a chat feel free to Trust me an email or reply in here. I'll do what I can. All the best, and do let us know what you decide.
Often OEMs want to push or publicise a certain model, hence why you saw nothing but Nissan CashCows everywhere a few years ago. 