In arriving at your layout figure weights to balance one another, also consider the effect of tanks both empty and filled. Aboard smaller boats one-room layouts make for coolness. Berths may be curtained for privacy nights or stateroom partitions formed of wire mesh panels. Curtains lower over these at night to give privacy while during the daytime the air has a full sweep through the cabin. If you decide to fit your boat out with house furniture buy something suitable, do not make shift with castoffs from home.
Maple looks well, is popular and fairly low priced.
Built-in fixtures always look more shippy, though. Berths and settees can be built to take stock size mattresses, wardrobes and lockers can be worked into corners, although chests of drawers are more difficult. I would suggest buying the latter, also ice box, medicine cabinet, and desk, then building in about them even covering doors and drawer fronts with the same wood as used for interior trim. Houseboat cabins are shaped to take these pieces, cruiser hulls are not. By all means plan the layout, figure on a heating arrangement and build your boat with the idea of being aboard a good deal during early spring and late fall.