I have a 2001 A4 2.8 that I recently bought in December. It had 120k miles and overall seemed to be well maintained with every single option available. I have done some work on the car already (coolant temp sensor, rear wheel bearing, coolant flush, etc) and I recommend looking for the following.
-Is the car leaking any oil? If so, the PCV system got clogged and the valve cover/tensioner gaskets blew. A guide to fixing the system is found in this link:
Updated: V6 PCV system replacement (now with suction pump!)
-Does the coolant temp sensor work? If not it is a ~$30 part and a decent amount of work.
-Any suspension sounds? The front suspension is notorious for having parts that need to be changed around 100-120k miles.
-Rear wheel bearings are a common wear item on the car.
-Has the timing belt been done in normal intervals (70-80k?) Did they do a complete service (Thermostat, Waterpump, rollers, belt)? Were they OEM parts???
After looking on the forums when I was purchasing the car, it seems that there are a lot of examples with 300k+ miles and virtually no engine problems. They are certainly more reliable than the 1.8.
Overall my advice would be that if you do like the car to buy the Bentley manual and a VAGCOM interface. Out of all the cars I've worked on (Subarus, BMW, Volvo, Porsche) the Audi seems to be the most difficult. Parts are fairly inexpensive.
Here are a few helpful links...
AudiWorld Tech Articles (Resources -> Tech Articles)
Audi A4 B5 2.8L V6 Timing Belt Replacement | Removal And Installation | | Happy Wrenching