Suggestions for a stiff boat
I have a Quoddy Pilot sloop that is very stiff. Cannel Paige and Paine have a Ketch version of the same boat on thier web site, it may be called an Eastport Pinky. I am not suggesting you buy it one but it does fit your requirement for stiffness. Some Catboat designs are stiff as well, you might give them a look too.
There are a number of traditional designs that are stiff. Also gaff or lug rigs will put less healing arm on your boat and you should look closely at the advantage they provide to some one who does not want to heel.
Look at them and read up on the subject, Ted Brewer has written some interesting material on comfort and the MacNaughton Group has good material on thier web site. In design terms you are looking for a boat with high initial stability. Of course multihulls have the highest initial stability of all and if its really important you should give them a close look.