I don't know much about the J10 Autodelta specific modifications, but the basic spider thins would be the same.
Roof - check for smooth operation.
Suspension - have a look to see if the car had had any suspension work, the rear bushes have a tendency to wear, the cost can be a little scary
Engine - Check the service history, when was the last cam belt changed, if it is more than 3 years/30K then it will need changing
Variator, this will mate a 'diesel' type clatter on start up, changing this will require the same level of work as a cambelt change.
Check the oil level, the Twinspark engine does not like being run at low levels, if the oil is low then there is a chance this has happened, there have been instances of engines going bang as a result
Body - The Spider/GTV has a galvanised body and so you shouldn't see rust on a car like this, check the front clamshell for damage, but remember that there will be stonechips etc.
If the car has a bodykit, what sort of condition is it in, getting replacements may not be cheap/easy.
There is a buyers guide at the top of the page that might be worth having a look at as well to give you a more in-depth view