A beach-cat will not take the extra weight of gear and supplies, especially for three to four people!

The Reynolds 21 or Stiletto 23 are closer to what you'd need and can be found a reasonable prices. The Seawind 24 is a boat designed for "expedition" style sailing and might be the best production boat you could buy for this purpose.

Otherwise, look at www.sailingcatamarans.com especially the Janus and Strider. These are boats you can build out of plywood and fiberglass.