I would definitely recommend renters insurance - without it, you could be very badly stung in the event of an accident, both for 3rd party liability and for the hull deductible. I have bought this insurance a few times, and have not found anywhere that will offer it for less than a year, although some will give a 50% refund if you cancel before 6 months after the policy starts.
You'll also need an FAA license to fly an N-Reg aircraft over there without an instructor - have you looked into the process of converting your JAA license, or do you already have an FAA one?
If you're only doing 5 hours, its probably a lot easier just to have an instructor on board and not worry about the insurance or license! You'll probably take at least 2 hours just to do the rental checkout, what with the new airspace and rules, and the new license.