Just have a thorough look over the car and ignore anything the seller says by the sounds of things.
Have a good look at the sills, front wing bottoms and rear quarter bottoms for rust, take a torch and have a look at the inner sills via the vents in the B pillar/door shuts (remove the vents). General checks around the car, funny noises, vibrations, electrics working (check sunroof operation), etc - check for smoke, oil colour/condition, etc - usual stuff.
Also have a good look at the paperwork, looking for good recent history. When were the belts last replaced, has the clutch/engine mounts/fuel or brake lines been replaced?
If you can get the MOT test centre number from an MOT, check the MOT history online.
Might even find a local, knowledgable member to give you a look?