Ah the (Audi) a3 turbo is basically the same mechanicals under a different shell. Same brakes/engine/gearbox/etc
The later model a3s (2001 onwards) got a few updates the golf didnt get (:D - Traction control, drive by wire throttle, Variable valve timing, 'electronicly lockable diff') and they make a little bit more power when tuned because of the VVT.
I have an Audi a3 and my brother has an mk4 GTI, almost every part is interchangeable except for the interior and body :)
Either way, you've made a good choice lol.
Oh and my email is sovereign_01@hotmail.com