In my opinion I would never own a DI model, the rfi are better, what you are talking about here is fuel injection over carb models, for instance a 2000 gtx listed as that would be twin carb model and 2000 gtx di would be fuel injected of which the di's where not very good. Then there are the fuel injected rfi models.
Now in terms of mixing oil, the years you are looking at would all be oil injected where this means so long as the oil pump is working you simply put the correct oil in the oil tank and it mixes it for you.
Now a 4 stroke does not need this it will have oil in the crankcase but then remember you will now have to preform oil changes on a 4 stroke where as a 2 stroke you do not.
Next a 4 stroke is going to cost you over $4500 in good running condition, you can get a 2 stroke gtx which is the 3 seater for around $3000-$3600 depending on the condition you can look at that where as $300 will get you a good running 2000/2001 gtx 2 stroke 951 with around 150-300 hours on it but wont look like new, then up near 3600 will get you lower hours usually under 120 hrs and the ski will still look like new and so will the trailer, you will always spend more for a better ski. Anything under $2800/3000 your getting into problems unless it is an older ski, but even then I have seen 1997/98 gtx's selling for near $3000 as well. There is a lot to look for while buying a ski, the best thing to do if you are unsure is to at min hire a pwc marine mechanic to go with you and inspect a ski so that you can be certain it is good, as well always and I mean always request a sea trial before you buy it, if there are no problems the person will usually say thats fine without skipping a beat, now if they change the subject, or think about it or give you an excuse then it may be best to go look at another ski, you will know because they will try to not let you leave as well.
The best thing to remember is to know everything is always sold for a reason whether good or bad......
Now in terms of maint on them it would be best if you wanted to learn on here it would save you alot of money, shops actually in my area are all usually over $100 per hour for a quality mechanic if they own a shop if they are fully mobile like myself you may find a little cheaper to save on overhead where as still $80 per hour.
The one thing to remember is a boat is nothing like a car, if it floats and has electronics and or a motor you are going to ALWAYS spend money to keep it going. Bosts are a money pit, I am not telling you this to scare you off just letting you know, with that said it might be better to buy a newer lower hour ski in better condition to keep down repair costs for now.
When it comes to boats there are two of the happiest days of a boat owners life, the day he buys it and then the day he sells it.
We are all here to help each other and can guide you through just about anything marine related.
