Hello mate.
The Mk5 GTI is a great car, however like most motors there are a few things to take heed of.
1. The Front arches rust (due to the arch liner rubbing, VW generally fix this OUTSIDE warranty)
2. On the TFSI Turbo, both GTI (k03) and Edition 30 (k04) need to have the cam follower looked at or replaced, it's a £25 part and can be DIY'd or indyed for 30mins labour. Revision C (latest)
3. If you feel a lack of boost at any time, things to check are the Dump valve (DV), get it swapped out for a Revision G (best) again about a £25 part and 30 mins labour :) see these things aren't so bad.. can be DIY'd too. Then the PCV (Positive crank case ventialation) introduced many moons ago for emissions; another cheap part and replacement.
That's pretty much most of the bad Mk5 aspects.
A decent stage 1 tune should see around 240bhp/280lb/ft+ on the GTI, and 290+bhp similar torque on the Ed.30
A nice mod is to remove the pre-cat which should free up to 10bhp with an appropriate custom map, pushing the GTI to around 250bhp... rTech are highly respected around here for custom maps, and iirc they are about £250! Bargin, with high quality service.
If your budget allows, go for the K04 Edition (ed) 30, it can easily achieve 300bhp with little effort... but then you want brakes... 4 pots from the Boxster are decent and keep the same 312 discs for lightweightness... etc. etc.
Most of all enjoy the car matey, they are a nice little hatch!