You can also consider getting an existing bike or buying a new bike and adding an electric motor and battery. Much cheaper and very easily repairable if you do that.

So there are some motors you can buy and lace it with the wheel. Some companies that sell motors like this include bafang. I would have also suggested mid drive motors but the setup is much more complicated, as many parts would have to be changed. But the benefit of middrive motors is greater efficiency and better balance.

Also i think legally you can have up to a 750 watt motor. Any higher and it isnt street legal, but that being said, idk how easy it would be to notice a difference. Probably not that noticeable.