Most of the issues you mentioned are cosmetic and can be fixed in a weekend. The key is to get inside and see what's going on there... leaks, mold, and if the engine runs at all. If it's an atomic and it has been sitting a long time, you are looking at new fuel and carb cleaning and possibly replacement.
Other little stuff adds up -- are the cushions shot (because new ones will cost as much as the boat). Does it have sails and what shape are they in?
The beauty of classic plastic is that it can usually be cleaned and fixed up pretty easily. But you need to be ready to do the work yourself.
I 100 percent disagree about spending $500-700 for a survey on a $1,000 boat. Go with Warren's suggestion, but that Casey book and when you do get inside, tell the owner that you'll be there for an hour or two to see what's going on.
I think it sounds like fun! When I got my boat last summer, it was neglected and very rough looking inside and out but with enough time and elbow grease she is looking respectable now. In my case all of the 'big things' were okay (engine, sails), it was a matter of finding and fixing some rain leaks, then scrubbing, cleaning, refinishing and washing everything inside. There is a lot of personal satisfaction in that for me.