First, I would categorically recommend that you stay away from the 2 stroke machines. My experience is that the older 2 strokes are not particularly reliable. I have a 1996 SeaDoo GTX and I have had to replace the engine, fuel lines, voltage regulator, etc...
The 4 stroke machines are much more reliable. I also have a 2002 SeaDoo GTX and that has been extremely reliable. I know that the Yamahas have a reputation for reliability and that is also a good choice. In addition, I would opt away from anything used in salt water.
In general, if you have to spend a little more for a newer and lightly used machine, that will likely be cheaper in the long run...especially if you are not going to do your own mechanical work.
Jetskier
