All the 1.8's are 20v and the vast majority are turbo charged.
Within the turbo'd ones there are a few different engine varients depending on age, again most are 150bhp (and certaibnly none are less than 150 bhp) but some are drive by wire, have variable valve timing, 'sportier' turbo's etc - little tweaks that were applied as time went on.
cant remember all the varients off hand but from late 2000 onwards till about 3003??! the 150bhp engine was the AUM with VVT, DBW and a sportier turbo. Then there is the 180bhp anniverary edition, but this became the standard engine on the newest 'standard' GTIs.
AUQ engine perhaps?
There are far more knowledgable people than I about so no doubt they will help. Basically don't buy a GTI unless its a turbo, you can't really go wrong with any of them, but the newer engines are a little bit more tunable due to the bigger turbo etc but if you can afford it a 180bhp version would be the best.
Bet you wish you hadn't asked!
HTH