Mattelk,
I have an '03 Islandia with the 240EFI I bought used. It was used in saltwater in FL. I bought it in June and used it for the rest of the season in MS and loved it. It's a great platform and many people enjoy the boat and its amenities.
With that being said, if I had to do it again and budget would afford it, I would go newer just for the Rotax engines. The Mercury is a great engine, but like many in this forum will agree, you have to have two dealers to service the boat if you aren't mechanically inclined. One for the 'seadoo' side and a mercury dealer for the engine side.
My best advice to you is to find a marine surveyor and have the boat throughly gone over if you have no idea about boating and boating maitenance. It's well worth the initial cost. I have had to replace my wear ring and stator this off season ($1000 in parts due to being cast aluminum), as the combo of salt water and lack or regular maitenance resulted in corrision to the the aluminum by the previous owner. If I had hired a surveryor/done more due dillegence on my part, I probably would have known these issues before driving 12 hrs to pick up the boat in FL and probably would have negotiated a better price.
Like I said, I love the Islandia and other than the wear ring/stator have had no issues with the boat. It runs great and always gets looks on the water, as it is the best of both worlds boating wise. If you have any questions/concerns, feel free to ask as many here on this forums are very knowledgable and helpful.