Hi from being in a similar situation to you not so long ago, I will add a few points.
Try to get a BWA engine (think they came on the 2007+ model), rather than the AXX which comes on the 2005. Because on the AXX engine there is a timing chain noise which can cause problems, it also costs a lot to fix as you have to replace the timing chain, the tensioner and sometimes the adjuster.
There are lots of option extras, for me I would look out for: Xneon headlights, heated seats, Parking sensors (for me its hard to see out of the back from a 3 door), Bluetooth for music, cruise control and Monza alloys (they make the car stand out, but they scratch so easily)
Look for good quality tyres, in my opinion this shows if the owner has spent money on looking after it. Also get someone to scan the car prior to purchase to see if any faults come up. Lastly look for full service history, ideally a specialist and look to see if there are invoices of work being carried out with this; ring the garage where it was serviced and see what they say about the car. Also look to see if PCV, DV and cam follower ( these depend on the mileage) and look to see if there is rust. Make sure timing belt along with the water pump( mind you some garages say the pump doesn't need doing at the time) have been done at 60k or 4 years, which ever one comes first. Lastly ask what fuel the car has been run on, best to always use Shell V power or super unleaded as I heard it can cause head gasket issues if its been run on normal unleaded + you buy a gti you should use good fuel :)
There is loads of stuff on here from people who have asked similar questions, just do a quick search at the top and just write down points and take those when you go to view the car + mine
Good luck