+1 on doing a water test, and I suggest trying a sit-down first as well. They aren't as easy as it looks. Again, WATER TEST any jetski. There are issues you may not see and you don't want problems out on the water. Unlike a broken down quad, you won't be walking back unless you're Jesus.
Look up the specs on clearance between impeller and wear ring. A powerful motor with a bad jetpump won't get you anywhere.
I went through a jetski phase a couple years ago and did a bare-hull rebuild on both a Polaris 780 sit-down and a Kwak JS550 standup. The light and nimble 2 strokes are a blast and much more fun than the big and heavy 4-stroke boats of today. Someday I'll get back into it and build up a Yamaha Wave Blaster2. We've got awesome wave jumping conditions around here on the beaches. I:I