Things to look out for.
The windows dropping due to the plastic brackets snapping. Ideal if you find one with the uprated metal brackets. If you get a none AGU then keep a close eye on the coil packs. Rust can be a problem, get a good look all around the car, and underneath. Mileage isn't so much of an issue as long as it's been looked after and serviced. there are guys on here rolling in mk4's with close to 150k on them, and that's petrol!
The big thing for me has been rubber. Since buying my mk4 6 years ago I've had to replace pretty much all of the bushes etc on the thing!
Obviously a test drive is key. Make sure you start it from cold. If you hear a whirring noise at start up then the starter motor needs attention. It should run smoothly if it's a good one. Nice clean acceleration with most of the power from about 3,000rpm upwards. Get a chance to rev it on the road through all the gears, making sure it changes smoothly and the clutch isn't slipping at all.