hello,
rust isn't going to be a major issue with most mk2's but accident damage might be so check the panel gaps , look down the sides and over the roof for ripples and check the boot and slam panel for creases.
don't be fooled by nice shiney paint, get on your hands and knees and check under the car and around the wheel arches for rust.
engine wise, 120000 miles isn't a big deal on a well maintained car but on a neglected one it might be.
check the engine is cold when you first get there. when
you start it, (dont let them as they can cover things up) get someone to stand behind and check for blue smoke.
check for erratic idling or misfires when warm.
gearboxes can play up in a number of ways , some bad ,some not so bad. crunching between gears might mean knackered syncromesh. difficult to engage gears or accidentally going into reverse instead of 1st are easily fixed and due to work linkages.
expect worn drivers seats on most gti's.
lastly buy one of mine , i've got 2
