Just a quick reply, I'm sure others can add to this. We must get round to helping John write the buyers guide for TSP. Also worth checking previous TOPICS on this BB.
1) Turbo - should see no blue smoke behind you when you boot it, no rumbling/knocking noises.
2) Gearbox - make sure shift change is smooth and all gears are selected
3) Direct Ignition unit - if its on its way out there may be a miss fire under load at mid to high rpm
4) Idle speed should not vary much - if it does could need a simple throttle body clean or more expensive AIC
5) Check all electrical goodies and switches plus there lighting bulbs - its a soldering iron job to replace bulbs in switches!
6) Oil/water mixing - head gaskets do go
7) Timing chain/balance shaft rattles - see if they have already been replaced (chains, tensioners guides that is) - expensive if a dealer does it.
8) Usual body damage/crashed/ringed checks
9) Suspension is pretty bullet proof