A few things to be aware of:
Check cam follower condition - this is a replaceable tappet that runs the high pressure fuel pump via the inlet cam lobe - they can wear through and damage the cam and HPFP if not properly maintained.
Inter-cam chain and tensioner (in particular) can wear and need to be inspected regularly after 100k miles or if noisy and replaced.
Diverter valve diaphragm often split causing boost issues.
Also PCV valves fail as well as old hoses, cheap to replace though
Some GTIs do consume some oil. Later models are better from my research and in particular the Edition30 model, which I own, rarely has oil consumption issues. I use 5w40 oil and it doesn't use a drop between oil changes
Top mounts on mk5 Golfs are horribly designed and if they've not been replaced in 15 to 20 years they are a nightmare to get off if needing to replace the suspension. The 3 captive nuts on each side can become "uncaptive" and spin freely, meaning you cant get the damn things off and have to cut the nuts off in limited space underneath, or from the bolt at the top (not something I'd enjoy repeating)
I'm sure others will chime in with additional niggles, but those are most of the ones I've kept an eye on in my ownership.
If you find yourself with the budget to stretch a bit further, I'd recommend the Edition30 BYD engined GTI, much more potential out of the box and uprated in quite a few areas compared to the standard GTI engine. I am considering selling mine in the near future as I want to move back to a VR6, in case you're interested!
