Not much in the way of answers yet. The money you have should be fine.
I sold my 1989 S4 with 60,000 miles for this sort of amount about 4
years ago, and that was in great condition with a full service record.
I had few problems, so it is hard to say what to look for.

For that money, you should get a good S4 (which seems to be the
favourite model) or a GTS. Make sure the cam belt has been changed (it
is a 60,000 mile schedule, many do it at 48,000). This is an expensive
job. The problems I had were mainly electrical, so it is probably
worth checking that everything works. I also had some a/c problems
when I first bought the car.

Good luck. I loved my 928 for the six years I drove it. An under-rated
classic.