I drive a 1998.5 (yes, .5) A4 1.8T Quattro. I personally love it. With a GIAC chip its putting out more power than the 2.8 (17lbs of boost spiked) but if you lay off the throttle it still gets good gas mileage (20-23mpg).
Are you getting a 99 or a 99.5? The 99.5s have some exterior modifications, mainly one piece headlights as compared to the earlier model 2 piece. You can clear corner (with S-YO bulbs) and throw on some HID ballasts for a more modern look.
If you're looking at modding the car, make sure to get the 1.8T. Like I said, a GIAC chip (around $500 all said and done) will put you to around 205hp at the crank. Throw on an exhaust, some injectors and other small stuff and you can easily get to 230ish. If you really want some balls you can go BT (big turbo) and get in the 350+ wheel horsepower range.
Check out
Audizine if you want to know some more.
EDIT: Like Pete mentioned, make sure that the car has had the timing belt done. If it hasn't, factor that into the buying price. Audis run interference engines, so if your timing belt goes the valves will smash into the pistons and you'll need to rebuild your head. Around a $3,000 job compared to a $800 (with labor) timing belt job.