As MikeG says. You want to get yourself a BBS02 motor. Mid-drive motors, ones that stick on the BB, are superior to hub motors, ones that replace the hub, as they allow you do use your gears (with a hub motor gears only serve to get a comfortable cadence). As it can use your gears it’ll manage load a lot better. For a cargo bike, ignore any hub motor.
Fitting a BBS02 kit (I’ve done it, and will be doing it again this weekend) is pretty easy, but a bit of a faff with all the wires. The unit comes with chainrings in the 44/46/48T options, though I’ve found this depends on supplier.
One thing you should note: the BBS02 is NOT road legal. It can be limited, with some controller hacking, to be sort of road legal but it’s a bit pointless. There is an earlier model, the BBS01, which is road legal.
For reference, the law stands at: 25kph (15.5mph) speed limit with a 200W (250W for cargo bikes) power limit. The difference between British and EU law is the power rating; they allow 250W for all classes.
There are other mid-drive motor kits out there, which are also road legal, and they mostly come from the Chinese market. There’s a motor by “Bewo” which is also mid-drive and road legal (from what I’m aware).
However, it’s unlikely that anyone will be able to recognise an illegal motor as the power ratings are written in small writing on the bottom of the unit and can easily be removed.
One caveat with these Chinese motors is that they lack the superior torque sensor found in the likes of Bosch motors, but rely on a cadence sensor instead for the pedal assist.