Since your boat is a SIB this thread belongs here
Inflatable boats - RIBnet Forums
< Edit: Moved 
JK >
This thread might shed some insight of what you will be faced with.
http://www.rib.net/forum/f50/launchi...and-53686.html
Your boat is lighter than mine, but not by much. It takes 4-5 strong people to get it back up a Greater Bay Area California beach.
Since sand here is soft, if not round pea gravel, I built a set stainless legs and use 19" Wheeleez tires. They float over the sand with zero issues and are designed to carry 265#'s each. I figure my boat going into the water is pushing just over 500#'s. For coming out of the water I can remove my anchor crate @ 25#'s, the electronics tower for another 35#'s, and the fuel tank for another 20-45# drop in weight. I am in process of taking an electric mobility scooter and using the drive motors, controller, etc, then I bought atv tires, to drive my boat up and down the beach. It just is too much to handle on our beaches for a couple of divers.
You can not buy wheels rated to carry around 500#'s commercially, at least I didn't find any and I tried. It probably would have cost me less than building a set.
Mine do fold up and down easy enough and they come off quickly, but I leave the brackets in place. Well usually I am wearing a drysuit and just jump in the water to push them back under the boat before hitting the beach. If I don't want to get in the water I built a pulley system to block and tackle them under the boat.
