Welcome along. It'll be good to see another GT around here when you get yours.
I can't think of anything to look out for when buying in terms of faults, other than really low mileage ones may be storing up future DPF issues if they haven't had any decent runs. Also the ride on the optional 18" wheels may feel a little firm to some people, but is by no means uncomfortable. Expect more tyre noise with larger wheels too, especially if you have Dunlops. This is common across the whole Mk6 range.
There are plenty to choose from, so spec is going to be the only difference other than mileage and price. When I went out looking for mine, all I wanted was a red 5 door with 18" Vancouvers. I've since changed the radio for an RCD510 and fitted OEM bluetooth, which makes the car pretty much complete for my needs. Oh, and chrome exhaust tips from a Skoda Yeti. At £25ish, it's they best value mod you can do.
Haggle hard with the dealer, especially if you have no px. I chipped nearly £1300 off the screen price of my car.