All the Sea Doo skis use Rotax engines. They seem to be extemely reliable as long as you take care of them and use good oil. I have ridden a friend's Sea Doo XP and it was an absolute blast to ride. I think just about any jet ski is hard to climb on while bobbing up and down in the water. The XP wasn't too bad.
85hp is plenty to have fun with. I have also ridden a Yamaha 760 ski with the twin carb engine, and they put out about 85hp. I couldn't keep up with my buddy's 110hp Sea Doo XP, but the Yamaha machine could easily get up on plane and out of its own way when needed.
I'd rather have a Sea Doo or Yamaha ski if I was to own one. Polaris has made some decent machines, too, but the early models with the blue engines had fuel-starvation problems. This issue, caused by an inadequate fuel supply from the fuel pump, often caused one or two cylinders to burn up. You take your chance with some Polaris models.
As far as I know, the only thing to really watch out for when buying a used jet ski is all the obvious (what sort of shape is it in, how does it run, etc), plus to be sure it has a clean and clear title in your state.