Yeah, you'll see some diesels with GTI (red I or a silver I) - doesnt mean anything, random badging.
I think the standard 16" BBS look good. If you have a budget of £6k then take out £300-400 out of your budget for a remap to 170bhp (you will enjoy the power and get the benefit of an even better mpg!)
As far as what to check for, make sure the air con works, all elec windows, and no knocking when turning the wheel - common faults, not that there are many on Golfs of course!
Def can get an 02 reg GT TDI with 50k or less.