Martin,
you might want to take a look at the
svenson website, the most comprehensive collection of free boat plans of my knowledge. It's got lots of plans of all different kinds of boats, including some single steps and three pointers. When you've decided on a certain boat feel free to ask again, chances are that someone already has made a scaled down model plan.
I really do like the idea of a geared outboard drive. I'm planning to do one myself, so I would really appreciate if you made a building report.
Greetings,
Andreas