Besides all of the following mentioned, also be on the lookout for the following (I've got the GT TDI 140, 2007 plate):
Rust along the sills
Door locks / boot lock - make sure they all lock when key is pressed
Check over all four springs the best you can, and especially the rears as they are prone to snapping / cracking anytime after 80K if they are the original ones
Regular oil changes every year with the proper PD oil + cambelt change every 40K / 4 years
If an example is at 100K or over, see if the clutch + dual mass flywheel has been done - the flywheel may emit signs of wear if it hasn't already been done
If you can, try and stretch to £3500 as a lot of genuine & cared for examples tend to fetch better money than others. Late 2008, early 2009 MK5's & lower mileage examples can even fetch as much as £4000 at the moment but there's no need to spend that much on one of these in my opinion. I paid £3500 for mine in August (105K miles at the time, full service history with everything that needing doing had been done & came with a folder full of past paperwork).
Liam.