I haven't done it in Samui, but have rented a small jeep in Koh Phangan a few times. It's an excellent way to get around and see some areas that you wouldn't be able to easily get to otherwise. The last time I hired one was in 2010, 2000baht for 3 days. Insurance? What insurance? They take your passport as insurance!
Seriously, I had an issue with leaving my passport. Even said 'forget it', walked away for an hour before going back with tail between my legs. I tried to leave a photocopy, but they wouldn't have it.
I hired from a small local place (a cafe opposite the pier) in KPN. I would imagine there are more options in Samui, including some of the larger, but more expensive firms that might have insurance options.
Thanks so much lol... So you did leave your passport?
I may just rent it in Koh Phangan
So in exchange for insurance they take your passport. So if you are to get in an accident you pay cash I assume? Thanks again, I will probably check the place out that you rented from..