I've recently bought one and there are a few things to look out for
-diverter valve an be faulty. This can affect all gtis and is very easy to replace
-PCV issues but again not to bad to replace and easy to diagnose
- Seals can be faulty and let water in. Best thing to do is to get a bucket of water and splash it over the car then check for problems as well as check the cloth for signs of damage
- Cam follower will need replacing as a pre cautionary measure failure of this part can cause immense repair bills. This is cheap to replace
-Check the tyre tread pattern to make sure its uniform across the width of the tyre. Ive read in the rear insides become worn it can be as a result of bad rear shock absorbers
-If its a private sale make sure you find out if super unleaded has been used
- If DSG make sure its had the service done for the DSG box I cant remember which intervals they are but they must be done and can be pricey
Also do all the usual checks you would when buying a car.
Hope that helps!