Right.
I recently sold my 4.6 V8 Range Rover that was fitted with a multi-point injection LPG kit. I loved it. LPG around here is around half the cost of petrol, not quite, but close at approx 65p per gallon. My fuel economy went from around 18 mpg on petrol down to 13.5 mpg on LPG. This was calculated on fuelly, full tanks etc... over around 5k. My average cost per mile was 23.4p. Don't look at the fuel cost compared to the petrol though, it's the diesel you compare it to, as generally if you don't have the V8, you would be plumping for the diesel if you have any sense. Then the fuel cost is pretty much half, meaning I was equivalent of 27 mpg, which is probably a little better than a diesel Range Rover, but with all the benefits of a V8.
If you'e looking at a circa £600 PCM fuel bill, and roughly half it for LPG, that's £300 per month towards the conversion, and you will get your money back in 7 months (say 8 or 9 as it's not quite half the petrol cost due to slightly worse fuel economy, and slightly less than double the price for petrol).
Along with not having to pay out for a new car initially, second road fund license, insurance and any MOT repairs or whatever on a second, cheap car.
Of course, you may actually just want a second, more economical car also. Which is what I wanted. Hence why up until I sold my Range Rover, I had 3 cars. My "good" car (BMW 530d), my winter car (RR) and my economical car, which was a 2003 B5.5 Passat 1.9 TDI.
Now, I paid around £1k for the Passat. It was poverty spec, 5 speed, but with the 130 BHP engine, albeit with 140k on it. Chocolate cams are more the problems of the 150 BHP or the 2.5 V6 diesels of the same era. I personally wouldn't touch them engines, as they are not particularly fast, economical or reliable, and if the cams do go, you are looking at a massive repair bill.
Now, my Passat averages around 47 mpg or 13.4p per mile. This is "almost" half again of the cost to run the RR on LPG, so represents around a 2/3 - 3/4 saving on your current calculated fuel bill. And would therefore pay itself back in around half the time of the LPG conversion. Say 4-5 months. Not including insurance, tax etc...
I personally don't get everyones fear of diesels on this site. I have run MANY diesels, from brand new, to very old, with no miles on it, to having been to the moon and back (I once had a 309 with well over 200k on it), and I have NEVER had a problem that was related directly to the fuel.
To me, diesel just represents the cheapest motoring costs. I really do have to bite my tongue at all the posts on here saying "don't get a diesel, not worth it, get a 330ci instead, no turbo or dpf to worry about". Whether you do 6k per year or 50k per year, your almost always more economical in a diesel (obviously more so with higher miles per year, less so if your comparing a 1.0L banger to a 335d or similar).
I have never had a problem with a turbo or a dpf. And you get turbo'd petrols too, so it's not like it's just a diesel cost. Then there is the higher purchase price of the diesel, which you get most of it back as the selling price is higher on the other end. The only time this is not relevant is when you run said car into the ground, in which case, your almost certain to get better value from the diesel.
Anyway, 2 options there. If you must get a second car, I would happily recommend a Passat Estate 1.9 TDI. Cheap to buy, relatively cheap to maintain, economical, and a cheeky remap will make the world of difference to it, and "might" give better economy as well as performance. And ISOFIX bars can be retrofitted at a little over £40 per side.
Then there is the LPG route. It's also a good one, so long as you have enough retailers locally. I had one on each end of the commute, within 5 miles of each end, and one in the middle, but always held off for the cheapest one (closest to work), as if I ran out of LPG, I still always had 1/4 - 1/2 tank of petrol anyway.
BTW, my commute is just over 120 miles a day, so pretty close to your situation. With around 30 miles A-Road, 80 miles dual carriageway and 15 miles town driving. And have done it in all the vehicles I have mentioned.
So, TLDR:
LPG the V8 and pay it off in 8 months
Buy a Passat TDI Estate, pay it off in 5 months
Both great options, and well worth doing.