Re: What is the best bike to build to sell?
Grubee GT1 or GT2. Both are built on frames made for motorizing complete with a drive gear on the rear wheel attached to the HUB, not the spokes. They have three brakes (2 v-brakes and 2 drum on the rear) but I'd remove the drum on the rear. The GT2 has a top tube gas tank built right in. They're a little pricier, but work pretty good.
Another good one is the Greenline BC-706m 7 speed. You won't need the extra speeds with the motor on the bike, but the bike comes with good v-brakes and is easy to motorize. The frame is a nice fit though you will have to fabricate longer straps for mounting the gas tank as the top tube is a squashed oval and the stock button brackets won't fit (I used pipe hanging strap doubled over).
If I'm not using the Grubee frames I like using multi-speed cruiser frames mainly because you need multi speed before the manufacturer puts good brakes on them. Single speeds always seem to come only with a coaster brake. That said, you can modify single speeds by adding U shaped cantilever brake adapters, but the extra cost of these parts usually prices them over the similar 7 speed models.
My first motorbike was an Electra Coaster 7 and I had to do a lot of work to get the motor mounted properly because of the extended shape of the frame. Typically Electra frames are NOT "V" shaped. They're more of an extended "U" with a flat section at the bottom. This will require extended front or rear mounts and probably a vertical strut to cut down on vibrations. Sure is a nice ride, though.