I don't know a lot about the Islandia, but my Utopia runs the Merc, and I have never had any problems. As far as cost goes when working on an outboard or the Optimax they will be the same due to the fact that the Optimax is just that, a V6 outboard power head.
Power wise that Islandia will be a rocket ship compared to a pontoon with a 50hp so that would be a big bonus.
The jets take some getting used to in handling, but once you get used to them you wont look back.
The boat has been sitting for over a year, after talking to some other people there seems there could be a problem with varnish build up and/or internal engine problems. Since I can not test drive or have anyone really look at it before I buy it and repairs can be costly, I am kind of leary of buying it. At only $4500 including trailer it seemed to good to be true and maybe it is. thanks for your input but since I do not know much about boats, maybe this isn't the one for me.