The answers will vary significantly depending on island and town.
Gilford islanders have use of the Glendale town docks and auto parking. The Rattlesnake Island Assoc had an auto parking area, someone from that island can speak to that. As already suggested. a realtor that knows about the islands can help a lot.
You will need an island boat, one than can carry passengers and lots of "stuff" in a variety of lake conditions. You will need to own or rent a slip, not easy as demand and prices are high. You should be able to park at least your auto at the slip site.
Island Support Service for one can get large items to the islands.
http://islandsupportservice.com/
You would be well served if you knew how to make basic repairs to plumbing and electrical systems as well as having basic carpentry skills and tools.