I'd make sure the car you're looking at hasn't been tuned before. Definitely buy a warranty for peace of mind. As mentioned, check the condition of the brakes. I had to replace mine at 45k. Of course check the car over for any damage, make sure its been serviced regularly.

I'm about to hit 52k in my RS7, I've had it over a year and I don't plan on selling it anytime soon. I say that only because I have an extended warranty - even though mine has been pretty solid, when something breaks on this car it is expensive. The turbos are inevitably gonna go at some point, unless you replace the oil screen, but since I have a full warranty and I'm not tuned, I'm not too worried. When it happens, it happens. If you don't buy a warranty then replacing the screen is very important and money well spent. My Rear Diff went out at 48k randomly, and I was told that would've been an $8k repair if I didn't have the warranty.

Enjoy the car!