Hi jickham,
This will be a real adventure as much for the soul as it will be for the gear that you will need. I agree with wayno that leaving the gear in the car is a doubtful option for a whole host or reasons but primarily leaving a car unattended for 3 months is asking for trouble, also agree with the posters that you need to read up on other peoples experiences.
My curiosity is with where you plan to do it, is this an Australian based idea or is the world your oyster when your planning. I seems like this sort of thing is done more in North America and Canada but there are some really remote places here in Oz. Have a look at some of the remote islands that are around Australian waters that are seldom visited. This sort of option would work with your boat in with all your supplies and then set up your camp scenario. It would also grant you that wilderness that you seek without interruption and as you said you could just have a planned extraction deadline.
What to take will be very dependent on where and considerable research will be needed on gear that would be suitable for your destination and it will also depend hugely on the time of year. Winter in some of the southern islands would be very dangerous, but could be done where as "doing" winter in the tropics would be more comfortable. Given that your time line could be out to three months you will not have a chance to propagate food sources and your capacity to supplement from your location would need to be well thought out.
The only non-negotiable for me and I am sure many here, would be the need to have a very resolute plan if something catastrophic occurs. Who and how you seek help, and what your expectations are with regards to rescue. I also assume that you wish too return home after the adventure to tell us all about it. So whilst your purpose is removal from day to day life, you still need to have the capacity to report on your well being at designated times as well as potentially coordinate your rescue if need be. Don't just take a PLB and push the button if your feeling lonely.
Also the sort of destination you seek is like it is for a few reasons, its hard to get to, (if your in trouble it may be days or weeks until help can arrive), its climate is not suitable for habitation or it may not have sufficient essentials such as water for extended stay.
Here is something that I would love to do.
My dream destination is here,
http://www.heardisland.aq I would love to go here in summer for 3 weeks or so, and "drop out, embrace all that it is and endure" and then come home.
Good Luck with it all, can't wait for the trip report.
G_U