You have a lot of studying to do.
Hub motors are available in different "windings" (how the copper coils are wound inside the motor) that allow you to chose your max no-load RPM. Generally, the lower the max no-load RPM (at a given voltage) the more torque the motor will produce. Also note that a planetary geared hub motor will generally produce more torque than a direct-drive hub motor.
The essential components of an e-bike system are motor, motor drive electronics, battery, and controls. For your application, I'd suggest mounting the battery and the motor controller in a small Pelican case on a custom front rack. That will help keep weight over the driven wheel. And, if done right, would make it pretty easy to remove the whole system with a minimum of fuss. I've seen this done on DH mountain bikes and it's a pretty clean solution. The alternative is putting the battery and controller in a bag/case on a rear rack. Not a bad approach, but you'll end up with less traction in front and you have to run the wiring harness the full length of the bike.
Off-the-shelf throttles come in three flavors -- full grip twist, half grip twist, and thumb. Depending on your fabrication skills, other variations are possible. High-torque motors tend to be difficult to modulate with the throttle, so you might find a simple on/off push-button is just as good.