Since it's been sitting... I don't think you will be taking out for a test ride. (the only way to know if it is actually working properly)
If it was me...
1) I would look at the general condition of the engine compartment. (clean is good)
2) I would take a compression gauge with me. If the guy is a mechanic he should have one. You want to see 140 PSI or higher, and the pressure between the jugs should be very close to each other.
3) look at the impeller and wear ring.
4) ask him what oil he is using. He should be able to tell you without thinking about it. There is only a few oils that are approved to be used in a SeaDoo engine. If is answer is anything other than Bombardier XPS, Quicksilver PWC, or Amsoil Interceptor... walk away from the deal. (unless it's very cheap)
5) If everything to that point looks good... I would still want to hear the engine run. It doesn't mean it will be OK... but it will let you know the electronics are working.
Good luck... let us know what you do.