I have a 1999.5 Audi A4 Quattro with the 1.8T and Tiptronic(Auto) Transmission...I have an upgraded K04 turbo/giac chip on mine.
With that being said, I love the car....for a sedan, it's a blast to drive- but if it was my only source of transportation, I would look elsewhere. In addition to this car, I have a newer Jeep Wrangler and a classic Chevrolet Camaro that I do all my own work on, including engine swaps...but working on this thing is another story.
These things are expensive to maintain, everything has to be ordered from Germany and requires a special Volkswagen Auto Group tool to do anything, so only specialists and dealers can service them (and they need to be serviced alot). However, it's a decent car as long as you understand that outside of a warranty, it's going to cost about $2k a year to keep it on the road...which is also true for newer Mercedes-Benz/BMW vehicles as well
If you are still set on the A4, try to find one with a 5spd manual transmission and the Quattro AWD system, it's really what makes these cars shine, being able to haul ass comfortably in any sort of weather (sun/rain/snow). As far as engines go- both engines are decent.... 2.8 VR6 is good for keeping it stock, 1.8T is great for modding potential. Both require premium fuel and need to have the timing belts replaced religiously on schedule or you risk destroying the engine. Other than that some common problems to look for are: lower control arms that wear out and start to squeak and some ball joints in the front end steering assembly(mostly 96s and early 97s). Faulty ABS pumps and side airbags sensors that malfunction are relatively commonplace, be prepared to deal with both.
Don't even consider anything with more than 100k miles, it will nickel and dime you to death