As already said rust open the tail gate check where the loom comes through they bubble bad there,lower quarter of the front wings,feel the arches as its hard if they have a arch kit on.Check the drainage bungs in the floor give them a poke usually always rust and then look at the sills near the front where gimps have used a trolly jack make sure the sills are not crushed in also check where you put the fuel in.Check the abs light and airbag light come on and go off if they dont light up at all someones pulled the plug.To be honest the rust is the main thing really as most other things are a simple fix I always reverse a car aswell as mk3s can suffer with gearbox issues but good luck in finding one if you do find a decent one there good motors..
