Be careful of the high mileage 1.8t's. I had a 97 audi a4 1.8TQ and at 103k the engine seized, and after 4k in repairs at MAG, it was never right again. It was a fun car, but they are not reliable high mileage. I would go with the V6, they are very stout and good performers. Of couse not so easy to gain power because its NA, but very nice either way. I've seen a4's of that bodystyle, 5spd, quattro, v6 with around 100k going for 5k and up.
Good luck with what you get.
Other nice cars are Passats, 98+ can be found Cheap, and are essentially the same car.