Welcome.
How much is it?
What options does it have? e.g. Xenons, 18 inch alloys, Sat Nav, Multi function steering wheel, Heated seats etc.
IF possible get the car scanned to see if there are any error codes, which will tell you what needs replacing if anything.
Look out for rusting / bubbling front arches. Check for cam belt / water pump change.
The 2005 GTI was a 2.0 Turbo TFSI using a K03 turbo, maps to ~250bhp on stage 1, later on 2007+ iirc, they brought K04 Edition 30 maps to ~300bhp stage 1.
Common TFSI issues, PCV / DV valve replacement, not overly expensive (if you loose boost these are what to look at)... and Cam Followers should be checked every 20k or so, sooner on mapped cars.