I am not any kind of electrical expert, but I have been reading for a while about electrical-assist bicycles.
If your interest is in e-motorcycles, e-bikes, and e-scooters, there are many helpful enthusiasts at:
http://visforvoltage.org/
Starter kits (for bicycles) often have a modest 250-watt/400-watt motor, and use 24 volts (two 12V lead/acids) to keep the price, size, and weight of the battery pack small.
It appears that anyone who likes it, and sticks with it, quickly graduates to a 500W+ motor and 36/48 volts.
With 48V being the desireable upgrade, keeping the battery size/weight down often means expensively upgrading to NiCD's or Li-ions.
Since batteries wear out and have to be replaced, and motors are well-known to be very durable, I thought that if I payed extra for a very big motor in the beginning, it would be a good upgrade (Etek-750W/1HP). I then read that a big motor draws many amps, which shortens range, so you should seek the smallest motor that "just" meets the loads you will put on it, without getting hot. Its difficult to calculate, because it depends on how steep your hills are, how long the hill is, and how many hills you have to hit on your route.
I live around frequent medium hills and am now considering a Scott-600W using 48volts. Bike riders who live in fairly flat regions have often reported being pleased with 500W/36V.
Higher volts means getting the same power from a smaller motor, which draws fewer amps. This will translate into less motor/controller/battery heat, and more range.