I know that the bike in the picture looks like it would be a nightmare in terms of stability, but you'd actually be quite surprised at how stable they are. Unless you're taking hard/sharp turns, it's perfectly fine at speed. I've had a wheelchair up to 20 mph before, and I've taken my handcycles (the lower type) up to 60. I'm confident that I would have no problem at 20 mph on this thing. I just wouldn't want to go much faster unless I had a disc brake installed. The manufacturer's (a medical equipment company that is ALWAYS worried about liability) literature even says that it's capable of speeds up to 20 mph.
The original drivetrain on this thing is pretty crappy. It has a wonky coaster brake setup as well as a 3-speed geared hub and 5-speed freewheel. I'd like to stick with the plan of just running it as an electric-only vehicle. I'm in New Mexico, and laws in regard to electric vehicles that do or don't have cranks probably aren't even in existence. If they are, they damned sure won't be enforced. Out here, it's still very much the "wild west." No one with a badge is going to give a rat's fat @$$ about some dirty dude on a quasi electric wheelchair that is going back and forth to work or the grocery store. Now, if I were driving it down Route '66, that might draw a bit of attention.
Anyway, I'll continue to do a little homework and see if I can figure out what would work best for this setup. It'll be a rear hub motor (as the front/drive wheel on this rig has 135mm spacing). I'm leaning toward a Nine Continent hub as they seem to be made to higher standards/tolerances. I'm very excited about this project.