Hi mate, welcome to the forum! Don't worry, it's been a bit quiet on here lately.
Check the front arches, sills (where they meet the front wings), and behind the boot badge for rust. It is unfortunately a common problem on the mk5. VW do repair or replace the front wings under warranty, but under certain conditions, mainly that the paint is original. You might not have as much luck with the sills or boot badge.
Make sure the turbo isn't excessively noisy, and that it's boosting properly. There are a few things that VW don't class as serviceable items - the dv, pcv and cam follower - that often fail. Symptoms of a failed dv in particular are loss of boost, hissy turbo etc.
Coil packs fail, just like the 1.8T.
Check it idles smoothly, hot and cold. The TFSI is prone to carbon buildup on the inlet side which can lead to lumpy idle and reduced efficiency after a while.
Try and find a car that's been on short life servicing, so once a year/every 10k. One that's had the belt and water pump done at the right time too.
If you can stretch to an Edition 30, they're the one to have (especially if you're considering tuning; they have the larger k04 turbo and stronger engine internals than the k03 GTI).
There's undoubtedly loads more to check. The search function isn't always great on here, so I'd suggest typing mk5golfgti.co.uk buying guide (or something similar, you get the idea) into Google.
Best of luck.
