Have a go at the search function for how too's at top of page mate but heres a few things to get you started -
Rust around front wheel arches
Test drive a few as many may feel sluggish due to failed turbo Diverter valve
Ensure has 2 sets of keys as around £190 each from dealers
Decent brand tyres are a must on the Gti especially at this time of year!
Full service history along with timing belt every 60,000 miles
Standard service plane is 10,000 miles long life is 20,000, best to try find one thats been on standard plan but not the end of world aslong as been done on time
Engine should feel responsive and smooth, brakes should be sharp
Plus all standard checks like hpi clear, any panel/paint damage
Clutch not slipping when opened up in 4th/5th gear etc etc
As said search function is your friend