This is one of the smaller motors one can buy for a DIY ebike and would add about 5 pounds. I put these in our folders. They weigh about 35 and 40 pounds.



Folders are now available with the battery in the seat tube.



While the above bike is probably too heavy, there are other nodels that are lighter, so I think the OP could be a bit late to market, even if it is only a study.

I could buy 100 Watt-hour 36V2.8AH packs from Grin technologies. Basically ten 2800ma lithium cells in series. but quite modular. Two in parallel would be 200 watt-hours. My wife and I use between 6-8 wh/mile on a summer day going about 12 mph. One of these sticks could give us a 10 mile range. We don't use them though.