You might find https://www.pedelecs.co.uk/ useful.
I recently bought a second hand cargo bike (that I can't resist showing off) and converted it using a kit from https://wooshbikes.co.uk/. Maybe a kit isn't for you, but they also sell pre-built e-bikes.
Some things to decide on:
How many miles you will want battery assist for, big batteries cost a lot and you might not need one
Whether you're cycling up steep hills, hub motors can't generate as much torque as crank drive so crank drive is preferred in this case