In terms of getting it wrapped, no idea.
2 common known faults that I know of;
- Diverter valve was a know weak spot, worth replacing with the upgraded unit, available from main dealer (part number 06F 145 710G) or check out Awesome GTI below
http://www.awesomegti.com/car/vw/golfmk5/oem-2-0t-diaphragm-diverter-valve-06f-145-710g- Coil packs were re-called by Volkswagen I believe, so if you take your car into a main dealer they will replace free of charge so I'm led to believe.
Also worth mentioning, when you buy a car, check for obvious signs of abuse. If you're not sure how to test suspension, brakes, clutch, gears and engine then let me know. I'm no expert but I've seen enough cars to know a good one. So far my GTI hasn't let me down. Nor did my 1991 GTI (threw a hissy fit from time to time though). :-)
Personally in your shoes, I'd buy a car that's due service work as it'll be cheaper then you can spend the money slowly over a few months doing it up. I should of replaced the parts like brakes with the upgrade rather than stock parts but live and learn!
And finally, DON'T RUSH! There's loads of Mk5 GTI's for sale in the UK so a good one is not hard to find, so don't settle on the first one you see. Try them out, get a feel for good and bad examples.
I wish you luck in your quest for finding a GTI and joining the force.