I built our ebikes around regular bikes using a TSDZ2 torque sensing motor and an em3ev 52V battery. Total cost around 600$ + bike + tools. The battery is already assembled so no risk of setting your house on fire. The torque sensing makes it really natural (like having bionic legs), the whole thing has an open source firmware if you want to replace the Chinese made one (https://github.com/OpenSourceEBike/Color_LCD) and you can use any battery with it. With a 850Wh battery we get around 100km range.
There is also the cyber bike for which you can buy plans and build from scratch but that one is more of a motorcycle and you’ll probably need certification + insurance to use it legally.