First and most important thing is to avoid the cheap big wattage direct drive motors advertised on ebay and the like. The are very inefficient and need a large (expensive) battery, and aren't very good on hills.
To be legal the motor should be a maximum of 250w nominal continuous power. They all give much more than that peak, between 400w and even up to 800w or more. Unless you have very steep hills are a very heavy most of the legal motors should be fine.
Higher voltage gives more torque which can be really helpful; most motors will take it but you must be sure the controller is good for the higher voltage. It will be simplest to go for a complete kit from the likes of Woosh; you'll know all the bits work together well, and you'll get excellent advice and customer service.
https://wooshbikes.co.uk/
One difference between systems is torque or cadence sensors. Torque sensors sense power/pressure on the pedals and behave more naturally, but you always need to put in power even on their highest settings. Cadence sensors sense pedals turning (they mostly don't sense speed of turning) and will let you ghost pedal, where you need to keep the pedals turning but don't need to put in any effort. That can be important for certain conditions; especially if you are liable to get suddenly tired on the way home.
Mostly hub kits have cadence sensors and crank motor kits have torque sensors, but that is far from always the case.
The rules about throttles are confusing; they are certainly allowed for helping you get going. Probably best to discuss details with Woosh or whoever you buy you kit from. Most Woosh kits include throttles.
Good luck in your search.