For slow speeds, the 12 hp you mention should be plenty to drive a hull in the size range you mention. But you'll have to figure a gearing and propeller size to fit the hull that you find.
As far as the hull goes, I would look for something relatively long and narrow. Also, the bottom could be somewhat rockered, showing curvature longitudinally. These features are best for efficient slow-speed travel. That won't be a very common animal in aluminum.
Have you considered building a hull yourself, either of aluminum or out of plywood? You said you wanted a project!
