Running costs can be high, not least if you buy one without a warrantly which in itself can be an arm and a leg! I've run my RS4 for the last 18 months without a manufacturers warranty as I felt it was not old enough and the cost of adding it simply made my nuts hurt at the thought. I'll say this out loud, £2400 per year for my car, mileage, use!
It really depends how your dad will use the car. On a long motorway run you should be able to see 22-26mpg based on how soft you are with it, around town low teens and if driven hard single figures. It's a big V8 with an appetite for petrol so it is to be expected as is anywhere between 1 - 4 litres of oil in between services. Tyres have lasted well on mine, I would expect 15k - 25k on them, which is good for such power and weight and they come in around £230 per corner. Insurance depends on many factors but I'm around £480 per annum at 44 with full NC and clean licence (touch wood). Servicing is best done every 10K miles but if you follow the computer it's close to 18k, which is too long I feel. Budget £800 per and you should be well covered.
Disks and pads from indi with OEM parts are still close to £1K, for the front and £450 for the rears. They suffer suspension issues with their DRC dampers and you are looking £2K'ish to swap them out. Rear exhaust baffles seize needing new back boxes, emissions sensors and stuff related to breathing should be seen as commodities and most will lose power with age due to engine coaking. At 378bhp on a RR test mine was one of the more powerful ones and I don't think it is any quicker than an E46 M3....in testing.
However, when its a British winter day it is epic. It does understeer but on the road that shouldn't really show itself and you simply need to drive around it, on track it falls apart really, but on road, A to B it is exceptional. I've not driven a car this side of an Evo that flatters (and protects) the driver so well, it really is very clever and very quick rear world.
Bits are pricey. A new wheel is £600, find one less and let me know because I can't. Sat Nav is crap, seats can look tired due to people not knowing how to get in and get out off bolstered seats, I say that because mine are OK, I've done over 100k miles and I'm not a lightweight and I've been in 40K cars with worse seats.
A summary, but buy with eyes open, get it inspected by a good specialist and it should be a good, reliable if temperamental car. I am not aware of major issues and my car still has the original clutch, box, suspension, diff etc etc, but I tend to be soft on those bits.