To know what power level you need (and what size battery, which is even more important than the motor and controller), you'll have to define what specific job do you need it to do for you, and under what specific conditions.
What max speed do you need?
What max range, at that speed, do you need?
What terrain does it have to handle at that speed? (what is the slope of hills it must climb? what is the road type (paved, dirt, rock, sand, etc)?).
What is the worst-case wind it must be able to maintain that speed against?
What is the total expected rider/bike/etc weight?
Etc.
Knowing those things you can determine how good the bike you need must be.
http://ebikes.ca/tools/simulator.html can be used to do this, either with the existing setups in it, or using the custom options for them.