If you want to build a realistic looking ship with the standard Lego hull sections, its actually quite useless to add much interior since either the deck would be too low or the outward apeareance would suffer. Also you have to consider the sails and rigging which make a modular design in this size complicated.
The only solution would be a small sloop without a closed deck, but then its not really "interior", is it?
A less realistic design - like the official sets- can of course feature a rich interior and much playability, but it will always look like a toy, rather than a mighty man o'war
I would suggest to use at least 3 mid sections since the lenght at the waterline should be about 3.5 to 4 times as long as the maximum width.
If you prefer a very small design, you might want to use the smaller hull pieces, like in the imperial flagship or the armada lagship.
The parts you need can best be bought at bricklink.com. Its quite expensive, but you get simply EVERYTHING.
Hope this helps.