If in doubt, book an AA/RAC Inspection. Well worth it for peace of mind. As said above, the cambelt and water pump must have been changed with this age and mileage, so make sure that's been done and if not, get them to do it or knock 400 quid off the price.
Make sure the service history is full, and check tyre treads as well. If these are low, get them to change them. It's reasonable to get them to pay for mid-range, but what I did when I bought mine was pay the difference in price between mid-range and premium, so they put Pirelli P Zero Rosso's on.
The other thing to do is an extensive test drive, going hard around roundabouts and into corners, so you can hear any creaks or bumps or clanging noises coming from the suspension. Also, do some quite brisk 3 point turns. This will highlight any knocking noises coming from the steering rack or suspension. I had some issues with these things with my first Mk5 GTi as I didn't do this kind of test drive. My current car is like new, no noises whatsoever, and from experience I established this before I bought the car.
Check the sills and arches for signs of corrosion. If there is no accident damage, this can be done under VW warranty if you find any.
Hope this helps mate