Hey there,
Just got in VW's and got myself an 8 Valve.
Mark 3's have a 'cutting out' problem that some suffer from. Can be caused by a few different things, mine turned out to be a faulty coil, but others have been fixed with relays, Hall sensor(new Dizzy) and Crank position sensor. Worth giving car your looking at a long test drive to check for this. (check out huge 36 page Cutting Out thread on this forum!)
Some Mark 3's can be rusty, floor pans, sills and spots on doors, tailgates and around the windscreen. Check for any wetness inside boot and footwells.
Also make sure gearbox feels ok and give car a good long reverse to make sure car stays in gear. Jumping out of reverse could mean knackered box!
Apart from that all usual stuff to check for with old car, after good thrash any Mayo on oil cap/dipstick(possible head gasket), does it hold its temp ok, not losing oil or water, recent servicing, cam belt change.
Check all electric windows/sunroof working (prone to breaking), and good luck with it!
Loads of very clever bods on this site who I'm sure will have loads of good advice for you, I am pretty new to this!
If you dont here from anyone try posting a question in the general Mark 3 section and you'll get some help!
Happy hunting