951 is a great engine
I have a 99 XP with the 951 and I love it, aside from a current issue with the starter. It is oil injection, but not fuel injected. It seems to me that the less technical the ski, the cheaper it is to fix.
I agree with StRanger, stay away from any ski that is modified, especially one converted from oil injection to premix because usually whoever did it didn't take care of the ski well enough to keep the oil injection working correctly.
One other thing, all of the bragging about 4-stroke fuel economy is way overblown. Sure, if you ride WOT all the time on a 2-stroke you'll burn up the gas in an hour or so, but who does that? I pull a tube all day (5-6 hours) and still have gas left for next time.