Hi mate, welcome to the site. There are quite a few comprehensive buying guides on here, it's just a case of finding them!
But the few bits that spring to mind are:
Check for bubbling at 12 o'clock on front arches, and sills. You can get these done for free at VW, but it's not always straightforward.
Once hot, if the engine sounds a bit chuggy it might need a new chain. The TFSI has a chain as well as a belt.
Do a search for PCV, DV and cam follower. They're all cheap to renew, but if a cam follower ever wears completely through it means cam and fuel pump damage. Try and find a car that's had these replaced - they're not deemed 'servicable' by VW (just like the chain) so catastrophes do happen.
They're really quite good on fuel in terms of cruising, even with all the mods and mapped up.
The normal GTI, with the k03 turbo, will feel perkier than a Subaru at low-mid revs, but not as strong from 4k.
The Edition 30 GTI, with the bigger k04, is a quicker car when you can stretch its legs, but not as 'squirty' on b-roads.
They can be tuned to a max of about 290bhp and 380bhp respectively.
There'll be lots I've missed in terms of buying one - hopefully someone comes along with more info - but the usual rules apply.... Oil changes and lots of receipts point to a good car.
Good luck!
