There are several things to check out on these boats; the engine compression being #1 on that list. A new motor has 150PSI in each cylinder. Anything lower than 130 means a new motor at about $1,000 (DIY). Other things to check:
1. Wear ring around impeller - the gap should be less than the thickness of a dime. Anything more and the wear rings need to be changed.
2. Throttle and steering cables should move freely. If they don't, it will be an expensive fix, IF you can find the parts.
3. Structural soundness of hull - crawl under the boat and look it over. Feel for soft spots in the gel coat.
4. INSIST on taking the boat for a water test (at least 30 minutes). Many on here have not done that, only to find out they should have.
5. Check the boat for leaks. We are seeing the exhaust gaskets are starting to go bad and they have discontinued the part (although there are other options).
6. Don't forget to give the trailer a good look. The bunks should not be sagging and should have good carpet on them. Check the wheel bearings (hopefully they have bearing buddies on), winch, tread wear, and lights.
Take some pictures of the interior and exterior of the boat and post them on here and we can let you know what the cosmetic appearance is going to cost you to fix up.