The hull dimensions will be determined by the powerplant selection. Ideally I would like to find a lightweight 4-stroke engine in the 45 to 60 hp range, but this is in the awkward place between lawnmower-type engines and car engines. One possibility is a geo metro 3-cylinder engine.
Instead of having a separate lift engine I am planning to use a belt driven lift fan. Some type of automatic speed governor on lift engine speed would be ideal, but I haven't given this much thought.
After doing a bit of reading I decided to shift away from the extruded polystyrene insulation foam as a core material, and use a structural foam or other engineered core material. I'm still doing a bit of research to see what would best meet my needs, but so far structural foams are looking very promising.