Kahlil,
I would agree with you that an 18' beach cat is right if you plan on sailing with four adults. I owned a Hobie 16 for many years. Although it could carry four in light to moderate
wind and sea conditions, two adults was the maximum payload when performance sailing, or when the
wind was up.
As far as righting a capsized 18 footer, I would imaging you'd need at least two good sized people. I could not right my 16 alone unless a boat happened by and helped by lifting the
mast tip out of the water to let air under the
mainsail, or by pulling downwind on a
rope tied to the upper
hull. The Hobie 16 manual said that it takes 250 lbs hanging on the righting line to bring it upright. Perhaps the
manuals for the boats you're looking at will have
advice for you.