my contessa will be engineless. i'm launching next week so i'll let you know how it affects displacement and stability (i'm not expecting it will be very noticable).
in regards to stiffening the boat i concluded additional stringers were not necessary for cruising. i did plenty of research and thinking, but when you come down to it the generous layup combined relatively small panel size of 26 foot hull appears adequate. i think a lot of the talk about adding stringers is a band-aid fix for hunters and catalinas with thin laminates. my 1973 Jeremy Rogers boat has one longitudinal stringer at the waterline.
my advice is buy one of the boat types you mentioned and start working- you'll have PLENTY of time to consider what you'll actually do during the fitting out, but there is precious little time to waste before you take the plunge.
-cody