Hi and welcome to the SeaDoo forum.
I can give you a list of what I do when I get a new to me ski.
1. Download a manual for the ski, you can get it on this site if you join as a premium member, or just google for one.
2. Check the compression, it should be around 150psi, if less than 135 you could have problems.
3. If the ski has grey tempo fuel lines, change them, use black automotive fuel hose, replace or clean the fuel selector, open up the carbs. and rebuild if necessary and the minimum clean or replace the carb. filters.
4. While you have the carbs. off service the oil injection system, change the oil filter and the small injector oil lines.
5. Check the impeller and the wear ring, change the pump oil.
I know this sounds like a lot of work, but it's not really all that hard, if you've never worked on a SeaDoo you're looking at maybe a weekend's work and if you need help just yell, there's always somebody on the forum that will help out.
Lou