Hi Andy,
I'm just coming up on 1 year of ownership of my 2014 RS4 in Ireland, and I haven't had any surprises really so far touch wood.
I'd start with your last point, B8/8.5 - it's the facelift basically. However the RS4 was only released in the facelift version as B8.5 so that's all there is.
I'm not sure what running costs you are used to with the B5. There's the obvious one which is petrol costs. I'm averaging around 20mpg with mostly short trips. On long steady drives i'd get close to 30mpg but that's with no fun in between
Maintenance long term can be expensive depending on what age the car is, how many miles its covered and what's been done already. Also depends on where you get your servicing done. Lot's of people have issues eventually with the DRC suspension, sometimes it can be fixed, other times needs replacement. I personally have standard suspension so can't comment on that.
Drivetrain oils have specific service intervals - Front final drive 20k Miles, Rear Final Drive + S-Tronic ATF filter/fluid + Diff 40k Miles. Can be expensive if all arrive at once, especially if you service at Audi. Spark plugs at 6 Yrs/60k Miles. Other than that its pretty standard stuff.
If you drive it hard, brakes will obviously wear out quicker. Genuine discs are about £850-900 per axle, plus labour and pads. Expect roughly 2 sets of pads per disc set.
Mine is approaching 75k Miles, gearbox is still on original parts including clutches, no signs of slippage or poor changes so far and drives perfectly. I don't use launch control for this reason.
The only random issue I have had was an engine fault light from cold start. Turned out to be Cam timing actuator in the end. I replaced this myself at a cost of £100 for the part and some time. That's it so far! Best of luck
