What a fun project! What kind of range do you expect?
I'd like to do one myself but an upcoming home remodeling project is absorbing my cash reserves. There's a like new Giant Twist being sold locally for $750. I'm tempted, but the war dept. (AKA my wife) would pop a gasket if I dipped into the remodeling money. When/if I build my own, I would use a Workman Cycles bike for the base. They're industrial bikes, made in the USA since 1898, and almost bombproof. Pretty affordable to boot.
Industrial Bicycles and Industrial Tricycles from Worksman Cycles Factory Direct Store
Keep us posted on the build.