For a 100lbs capacity: From memory a single lead-acid 12V battery can weigh 30lbs or more. Depending on the specific trolling motor, you can easily add another 15-20 lbs or more! I think a single trolling motor would probably be more than adequate to move your pontoon.
Typically in North America, trolling motors are rated by thrust provided, in pounds. The bigger the number, the more power you get, and the more battery you'll need. As I mentioned before, some models do have a motorised steering ability.
Most modern ones are meant to be used with a foot control, either wired or wireless. The controls are propeller speed up, propeller slow down, turn left and turn right. You could replace the control pedal switches with some sort of relay. This could allow you to automate the propulsion...