Hi Stuart: I don't understand the "more expensive is better" given the other criteria. I'm thinking you want a mahogany-body double or triple O size, and they are not especially pricey.
Like this Guild M-20 (search for that down the page), http://www.12fret.com/used/index.html
--that one is sold but I think it'd be about right. Maybe there's more out there.
Martin made an actual Libba Cotten model, logically that's what you want, perhaps. Search for "Martin Elizabeth Cotten". It seems to be a 00-18 style, all-mahogany body. (Course hers was a lefty)
Don't worry about one not being expensive enough if it seems right for you--by the time you pay for shipping and duties, it will be expensive enough to satisfy anyone!