I will agree with SpectreChief, don't rush into anything until you learn enough to know what a good buy is. Also, if you plan on getting into the sport, you have to be committed to maintaining your machine. You can't just ride, put it away and assume all will be fine. Otherwise, it will be you rebuilding/replacing engines.
Here is a quick rundown of what I know about seadoo models and some other general knowledge to help you out.
GTI - 3 seater, entry level model, low HP, the slowest of the seadoo topping out MPH in the 50's
GTX - 3 seater, "luxury" model, HP ranges from low to high (50's to high 60's in MPH)
RXP - 2 seater, racing inspired, more nimble handling, typical high 60's MPH
RXT - 3 seater, again, more of a racing flare to them, the new model RXT-X does 0-30 MPH in 1.7 sec & 0-50 MPH in 2.9 secs.
WAKE editions - more or less GTX's with towsports features
No longer being produced
XP - older model, 2 seater, small and nimble racing inspired machine (new models are now RXP)
SP, SPX older models, I think they stopped making these in the late 90's and perhaps the SPX around '00
GSX - late 90's machine, the limited (which I have) is a medium sized 3 seater with lots of speed
A "Limited" typically just has a few more bells and whistles
I believe, that is most of them.
Engines
Wave runners were first produced with 2-stroke engines until the early 2000's. Now all of the newer models are 4-stroke engines. This was mostly due to environmental regulations.
The 2-strokes require a oil gas mix, but most machines have oil injection systems that "mix" it for you. Some choose to eliminate this system and premix their own fuel mostly to eliminate a point of failure (preventing one cause of a blown engine). These engine can be pretty picky and require a lot of active maintenance just to ensure they are running properly and to catch issues prior to them becoming "costly".
The 4-strokes are much larger engines, and much more costly to repair if something major goes wrong. Superchargers will give you better performance, but also, more opportunity for something to break and repair. I have yet to own a 4-stroke (plan to soon...RXT-X 260), but overall, the engine tend to be more reliable than the 2-strokes.
Water
Salt water is much more corrosive than fresh water. That is why people would rather buy a fresh water machine than a salt water machine. However, that is not to say that a salt water machine cannot be in good shape. It all depends on how good the previous owner was at flushing/cleaning it after each ride.
Hours
I personnally like to find a low hour machine. Chances are, most of the equipment on the ski is in better shape. You will find others say that sometimes a high hour machine is better than a low hour machine. So, just because a machine has low hours, you can't assume that you are safe from issues. Older machines that have sat for long periods will still have it share of potential problems. Don't just assume all is ok because it is relatively in new condition... go riding all weekend long... and end up with a blown motor. You have to do your due diligence and thouroughly inspect the machine. Make sure everything is in working order, ask around on the forums for common issues with that particular machine/engine and make sure you address them before you go crazy on it.
Also, don't be completely turned off by a machine that has had parts replaced or repairs made, carbs rebuilt, etc. These things are going to happen and normally, if the repairs are done right, you will again have a great running machine. Although, machines with brand new motors, barely or not even broken in yet you are going to want to be particulary careful with. There is a reason the previous motor was blown, and unless that issue is fixed, the new engine won't last long either.
Year
This does not matter quite so much. Find the other attributes of the machine that matter to you (engine, features, reverse, etc) and then view the styles of machine and range of years you narrowed yourself down to and go from there.
Hope this helps you out and good luck.
