Having bought mine a year ago, I would say:
1. Make sure the door handles appear to be working smoothly and freely (hinge can seize and snap needing replacement)
2. Check the condition of the disks (prone to degrading fast if left standing)
3. Check the gear change is smooth and the gear selection defined (Top Hat would need replacing if it feel like a stick in a bucket of mud, a minor job)
4. Listen for any whooshing sound when it hits 2500 rpm. Lower turbo hose prone to splitting. Again a minor job
5. Suspension squeaks and knocks. It's a fairly hard ride as it is but the suspension does not like being dry.
6. rattle from the hatch door. Can usually be easily fixed by adjusting the positioning screws on the inside of the hatch
7. Check the 'Bertone' badges are on the sides, if not could indicate accident damage repair
8. Has the car been sealed underneath. It's a good idea to do it.
9. Has the car been remapped and does the clutch slip when accelerating hard in 4th gear. If both are true an uprated clutch is needed.
10. Check all the switches, electrics etc
11. Check the tyres for uneven wear
12. Billowing smoke is an obvious thing to look for.
13. Check the history, has the belt been changed and the water pump too ?. Belts and water pump can cause nasty damage if they fail, but that's rare.
14. Do the doors open and close smoothly without any noise. If they 'Clack' a bit could mean replacing door straps, an easy and inexpensive job.
That's all I can think of, others may be able to add more.
If you do buy, you'll be hooked and just love it.... also a good 'Lube' (white lithium) will become your friend