Key points to look out for/test are;
Diverter valve - Hard to tell without jacking the car up on a ramp and removing the component. But the best thing to do is try and get a ride in a GTI that has been checked full so you get a feeling for the power and delivery then you can tell if a car you view is faulty.
PCV valve - The pressure control valve allows vaporised oil to be put back into the intake and burnt in the combustion chamber. This part has been revised as it was known to fail on earlier models. Way to test is to have the engine warm and idle, then VERY QUICKLY remove the oil filler cap (no more than second TOPS). If the engine goes to stall, it's all good. If it carries on idling normally with the cap off, the PCV is faulty. Cheap and simple to replace, but use it to haggle the price down!
Rusty front wings - This was a design fault and only affected some cars. If there's any sign of rust, fear not, for Volkswagen admitted this fault and offered to replace parts free and pay 50% labour. Some cases people have negotiated the whole job for free depending on dealer kindness.
Generally, check everything properly, ask the dealer to give you 15-30 minutes alone so they can't distract you from your inspection and take a friend as two sets of eyes are better than one. Do all the usual checks like service history and recites, ask questions and haggle them down as much as possible! There's loads of GTI's for sale so don't settle for the first one you find.
