Don't buy. That is, unless you like fixing boats more than sailing...

Additionaly to what was already said (straight mast, usable sails, etc.) old Supercats may develop problems in the transom because the rudder stock and gudgeon are strong and the glass may break first. Same with gudgeons loose from th transom. Old rudders may not be working. Bring a silicone spray and apply some to see if they move (one line pulls down and the other pulls up - test). The stainless steel rigging may be in bad shape, torn, etc.

It was stored outside, but maybe something was used to protect it from the sun. Check this. Exposition to UV destroys the tramp and make the hulls look ugly long before permanent damage is caused to the hull. UV damage may be only cosmetic, but it can also go real deep, sometimes beyond repair. Verify.

Somewhere in the web there was a checklist specific for each cat model. There was a link in the Catsailor site years ago, I'm not sure if it is still there. Try to find it if you have to check the boat without experienced aid.