It sounds like you guys really thought this through (with the LLC, etc), however I can't shake the feeling that you're going to find this a bit like owning a boat: it sounds like much more fun before you actually do it. Five acres is slightly more land than I have (in the exurbs of Toronto), and is almost certainly much more awkward, with less usable space. I mention that because I can't imagine trying to facilitate…
I can't see the appeal of this at all. Random list of why: Inevitable fights/headaches when one person wants out -- yes, you planned for it, but it's still a hassle. All the pains of homeownership (leaky roofs, pipes, HVAC problems, etc.) x 1000 due to difficulty of getting a construction crew to the island. Lack of a police force to enforce basic property rights against squatters/hunters/other "intruders" with diffe…
A that latitude, collecting rainwater and maybe sterilizing it is a plentiful source of drinking water