You could find a hull manufacturer to make you a hull and fit a console say from Windsor marine that would give you the option of a small cabin, provided the hull suppliers was prepared to fit the console and leave a hole though the deck inside the console to provide enough of a useful cabin.
It might be feasable to get the hull tubes and cabin for less than 20k but I think you'll be looking at a 7.5m rib as a minimum length.
I would have thought it was usefull to talk to the Windsor Bros, Osprey or Shakespeare for costs etc.
Looks Slow but is Fast
Member of the ebay Blue RIB cover club.