If I were to build an ebike and I was unable to ride a regular bike for any reason, I would begin with the basics.
Number 1: I would start with a men's bike. The frame design allows you to install things like batteries and controllers low down in the bikes center of gravity.
Number 2: I would make sure that all of the functions of that bike work correctly. Especially the brakes.
Number 3: Make sure that the bike won't fall over. I would fabricate my own adult stabilizer wheels using bike parts from the dump. I might duplicate the product sold by www.stabilizerwheels.com
(I have a welder, grinder, hacksaw, tape measure, and clamps to hold my work)
Number 4: I don't want anyone to steal my bike so I need a good lock.
Number 5: I don't want to get wet when it rains so I need fenders.
Number 6: I want to see and be seen if I ride in the dark so I need a headlight, tail light, and reflectors.
(I bought 2 - $10 big rechargable flashlights from Canadian Tire and fabricated a mounting system for them. The flashlights have both a floodlight and a three LED red light switch so they are interchangable for either front or back.)
(I bought 1/8" x 3/4" flat bar, 1/4" ready rod (redirod) and a couple of 1/4" self locking nuts from Canadian Tire, I found a plastic jug that was the right size to fit the flashlight, a scrap piece of iron pipe that was just slightly larger than the diameter of my handlebars, and I found a stretchy cord to hold it)
(I clamped the redirod to the scrap piece of iron pipe in my bench vice, and bent the redirod around it to make a u-bolt, the I cut it off with a hacksaw, filed the rough edges off with a flat file, and made sure that the nuts would thread back on. Make 2 u-bolts.
Drill a hole through the end of the flat bar just a little bigger than 1/4", then take your homemade u-bolt dip the shorter end in something wet (paint, dishsoap, oil or whatever) put the long end of the u-bolt in the hole and drag the wet end across the flat bar to mark the spot for your second hole. Drill it.
Got to go to work I will edit more later