Obviously I would buy one that has an hour meter so you know exactly how many hours are on it, and I've always looked at the engine bolts (ie: the bolts that hold the heads on will have marks (or tracks) on them if they were removed for some reason in the past) Just because it was taken apart sometime doesn't necessarily mean its a bad deal, but from my past experiences, people usually had not told me it was rebuilt or taken apart until I ask because of the bolt tracks.
Also, I've personally only owned 2-stroke jetskis, but I ride my friends Honda four-stroke turbo one in a while and I must admit its much nicer than my Ultra 150. So I'd highly recommend a fourstroke - their easier starting and more reliable (no fouling of spark plugs, etc.) I was at Tom Clark Motorsports in Belle Vernon earlier in the summer and they were giving pretty good deals on brand new leftover 08 models... not sure if they still have any, but just I thought.
Hope this helps and good luck!
Personally I like Sea-Doo's. I recently rode on a Kawasaki 1300, and man was that thing fast. I had a 1997 Sea-Doo SPX, which was a 2 seater and was 110HP. It topped out at 60 on a radar gun. I had sold it a few years ago because I really never used it. I think that if I would have had two of them, I would have used it more. If I had to do it all over again, I would get two of them. That is just my 2 cants worth.