My Audis Past:
1990 Audi 100 Quattro (5-speed) - bought used (2nd owner) with 30-something,000 miles. Had 230-something,000 mainly trouble-free miles on it when I got t-boned by a radio-tuning pickup. Still had its original AC, alternator, axles, PS. Other than normal wear items, I had to replace two fuel pumps, one multifunction stalk (lights, turnsignals & cruise), the driveshaft, ventilation fan and a couple of window switches. Not too bad for 200,000!
1991 Audi 90 20-valve (5-speed) - bought used (3rd? owner) with 95,000 miles. Traded in with 275,000 miles on it. Had to have the transmission rebuilt at 165,000 and it was going again when I traded it - a very unusual problem for these normally-bulletproof boxes. I replaced the driveshaft (common problem with that vintage Audi), the multifunction stalk and the alternator brushes, but everything else held up.
And Present:
2001 Audi S4 Avant (6-speed) - bought CPO with 42,000 miles, and now have 89,000 on it. This model is known to be fast and fragile, and Audi’s warranty has paid for a lot of little parts and will soon be paying for a turbo (leaking oil seal), leaking valve cover gasket and an as yet mysterious coolant leak. An O2 sensor and a motor mount went right after delivery while the fuel pump and serpentine belt tensioner failed earlier this year. While these problems would probably put it at the bottom of a Consumer Reports worst buy list, I’d buy it again in a flash - if one was available (Audi built less than 1,500 of these little beasts) - it’s that much fun to drive.
I also have two B5-series VW Passats - a 1999 1.8T (5-speed, bought as-is two weeks ago) and a 2002 V6 4motion (Tiptronic, CPO a year ago). They have most of the same running gear as an A4. The '02 has been mainly troublefree, though I did have to replace the temperature sensor (a common problem part).
The B5 Audis (A4s & S4s) and VWs have a reputation for “electrical problems”, though some of these are actually check engine indications due to too-tight sensor margins and short-lived sensors. You said you’re looking at a '99 “2.8T” (no such thing unless it’s a custom job) which I presume is a typo for a 1.8T. That motor has a very good reputation for reliability and balance between performance and economy - as long as you keep to the factory service schedule or better (I change oil & filter every 5,000 instead of 8,000-10,000, timing belts at 80,000 instead of 105,000). If you plan to work on your car you’ll want to buy a VAG-COM so you can talk to the car’s computer. You need it for everything from troubleshooting check engine lights to bleeding the ABS.
For more info (of every kind - good, bad, insane…) than you can possibly use, check out the A4 B5 model thread at AudiWorld.
Disclaimer: I might be considered slightly biased toward VAG products - YMMV