Jim,
I have an 08 Challenger with the 215 SCIC motor and from what I've read here the big question is the washers. Ask the owner if the washers are still the stock ceramic or if they changed them out the steel provided by Riva? If the answer is the ceramic are still in there that is an area you should focus your efforts first. Search the threads for topic of ceramic washers and you'll see why.
Other then that, changing the oil and cleaning the air filter and keeping things clean is always an indication of good maintenance. If you aren't sure you can always ask the owner to get a certified Sea Doo mechanic to give the boat a maintenance inspection. The couple hundred dollars could save you thousands in the long run.
This is MHO so other please ump in and add your thoughts.
Ted