Depends if it is an S1 or S2, look for rust on all removable pannels, doors boot bonnet etc, as you will need them for the rebuild.

If it's a S1 then parts are hard to ssource and can cost £££ for even little bits, S2 are more common so bits are easy to find.

Interior as that can be expensive to replace, so look for ripps or sagging or wear.

Gear box, good changes and no noises.

Engine, no rattles or knocking, no smoke on start up from cold or under boost. Clean oil and water with no contamination.

Make sure the turbo pulls well and the clutch isn't slipping.

Radiator leaks and is it alloy as you will want an alloy one.

suspension, standard stuff will be tired by now so has it been replaced.

Wireing and switches, does it all work, could be expensive to sort.

Is there any history and bills with the car, no use once re-shelled but a good indication as to the care shown for the donnor car by previous owners

If your reshelling it then are you getting a turbo shell, a truly amazing S2 was for sale on her and was stupid cheap because an XR shell was used so wasn't a turbo anymore. Remeber you will most likely loose the history on a reshell as it probvably wont come with a shell and you history from the purcheased donnor car is from another plate. This is when all bills and reciepts and a fully photo rebiuld is essential for when you sell it. In most cases shortly after the biuld is complete as you itch to do something else.

My best advice would to get a motor with plenty of goodies on it, the block will no doubt need a rebuild anyway but if you have all the samco hoses, chrome bits, alloy rad and intercooler, decent suspension, brakes, and exhaust etc then you have all the expensive essentials ready to be cleaned and dropped in and put it all into a turbo shell to regain some value when selling it on.