Steer well clear of the Mk4 2.0 "GTI" if I were you. In my opinion it shouln't really be called a GTI (115bhp and 0-60 in 10.5 seconds isn't really GTI pace). 2-litre sport would probably be a better title. I test drove one myself, and although its quite nippy and is a nice car, you'd probably be better off with a 1.6 16v SE which have similar performance and are much cheaper.

The 1.8 Turbos do offer the most bang per buck, especially the 180bhp ones, and can be fairly easily tuned to give well over 200bhp. Look out for Starry and Tophuh on this forum who have done just that. (I think the standard 150bhp ones do 0-60 in 8.2s and the 180's in about 7.5s).
How much are you looking to spend? Have you considered the 1.8 non-turbos? I've got one myself and they're pretty good (125bhp, 0-60 in about 9.5 secs). Got mine in December '03 for ?7500 from VW with 43000 miles on the clock. You could probably pick a 60000 miler up on Autotrader for somewhere in the ?6000's.
If you've got a bit more dough, think about the V6 4motion. These are lovely cars.

Common problems on the Mk4 include the electrics (esp. windows) and washer/aircon hoses leaking. Not had these problems myself... just what I've heard.
Hope this helps

Rich