Just recently purchased a TB42s mate and have been told some things to be wary of are head gaskets blowing or heads cracking (knock on wood), this is very often because people never adjust their tappets so the tolerances get out of whack and allow hot gases to stay in the head thus causing the head to say bye bye. Another thing to be aware of is the steering box at around 200-300 thou, they get sloppy and there tends to be a lot of play in the steering at 100k's. The auto trans is apparently 3-4k to rebuild, it's a complicated piece of gear and as a result can cost a kings ransom to fix.

Try this blokes website particularly the 'Buying a GQ' section. Very helpful

http://www.gqpatrol.com