There are multihull designs out there with good accomodations below 30 feet. But I'm afraid they might be expensive if you can not find 'the right deal'.
Most of them are designed for homebuilding, so you should consider a survey if you find a model you like.
Personaly, I like Richard Woods designs at
http://www.sailingcatamarans.com, especially the Gypsy and the Rob Denney design Harryproa at
http://www.harryproas.comIf you have the time, skills, money and motivation, you might even consider building it yourself..
