its an open ended question
I'm afraid that's your answer.
Unless you've carried out a decent inspection of the vehicle to find out what "really" needs attending to - i.e. how much you're prepared to fling at it - it's impossible to hazard a guess.
1. It would help if you can access its history/service record to see if it's been looked after.
2. When were belts last changed
3. How far has rust penetrated into the usual suspect locations, i.e. in the boot where it meets the wheel-arch seams, down behind the rear Pininfarina badges, and general floor condition.
It's all repairable but labour = dosh, so based on your research of similar age coupes and selling prices you could make them an offer bearing in mind the above, the worst they can say is no
There isn't a coupe around these days that doesn't need work of some sort or another, unless they've been well-cared for or renovated.