I'll second that valvers can be absolute sods to keep going and thats with a well cared for one. Make sure its had a cambelt change or you want £200 off straight away!! Check when the brakes and tyres were last done as they need replacing around every 30k and they are not cheap.
DRIVE IT!! as crezz mentioned some will of been cruised around in and never seen over 4,000RPM and they will be sluggy and when you start giving it some beans your likely to see issues such as head gaskets. Check for water in the oil and mayo on the oil cap, ask the owner what driving its done. Another major thing is the gear box, accelerate to a good 6,000rpm in each gear and listen for whines and whistles, syncromesh can be a common failure.
I've drove loads of 16v's when looking for mine and none of them ever felt the same, so get some 16v experience under your belt. Drive a golf if you have to just to get a feel for the engine and its characteristics, but dont buy it with out having a basis for comparison.
Final thing makesure you have a bit of cash left over as most owners on here will tell you they love to go wrong at the worst possible point *gives cordy a dirty look*.
There's probably more but my heads dead