Sensible option

: Buy clean Driver, keep it for 15 months (youll hopefully 2 years NCB), and try not to spend much on it. Save up
then get a mint GTI later. Makes more sense for the purse strings Im afraid, but then youll end up with a great GTI in super nick.
More Fun option: Bite the bullet, find the best possible GTI you can for your budget, and learn everything you can about them so if work needs doing you can get it done for less $$$. Become obsessed, spend every penny you have on it, and dont forget to use Optimax!!! Then start planning your mods/improvements.
GTI if you can!

Performance figures... i think from Haynes Max Power Golf manual
1.8 Driver: 0-60 = 10.1 secs, Top speed = 108 mph
1.8 GTI 8v (pre '88): 0-60 = 8.5 secs, Top speed = 118 mph
1.8 GTI 8v (post '88): 0-60 = 9.2 secs (umm not sure), Top speed = 118 mph
And Id hate to sound nosey, but youre almost 18 still a student, I guess if your starting work come autum you may be in a better position to run and maintaine the GTI, however if uni is your route it may change things (need of car, location, etc).
I ran a mk3 1.4 SE (fat and slow, but it served me well) for my first year to get 1years NCB and 1years experience, then moved onto GTI, but being 23/24, oh and married makes insurance cheaper.

My first Golf. 1996 mk3 1.4 SE... NOW SOLD

1991 8v GTI, Atlas Grey, 5-door
Nick