I've converted three 20" folders and one minivello. None of these were intended to be carried around or folded regularly. The smaller bikes were just more convenient to carry to far away places in my car, and my wife like their handling. A Q100H is a little less than 2.3Kg (5 pounds).
I've used the Q100H from BMSbattery.com, and the Aikema 100 from topbikekit.com. No VAT tax in the USA, so a motor is $120 USD shipped. I bought rims and laced them myself. About $200USD for a complete wheel. I had a problem with the Aikema coming in at 138mm O.L.D. with a 7 speed freewheel and my last frame was 135mm and too stiff to be sprung even 3 mm. I do not know if the motor was any different. but the BMSbattery print says their O.L.D. is 134 mm. I was forced to use a 6 speed freewheel on the Aikema . I woudn't get another unless I knew the frame could take it.
I find a 17A or 20A KT controller/display is nice. The square wave models are noisy. Whirrrr.
Two of the bikes run on small 36V10AH bottle batteries. I have managed to do 30 mile rides on them on flat ground, but we rarely go more than 20 miles.
The others use various softsided 36V packs, some custom made and others purchased, which are kept in bags.
I generally used entry level folders as donor bikes. WOuld be nice to have Dahon quality,