1. One should be enough, assuming you have an idler bearing on the other side of the scoop mount.
2. Time + Money.
3. Pololu has several good metal gear motors with various power, torque, speed, weight, and cost trade-offs. It's impossible to know which is right for you, because we don't know how big your robot is, how heavy things it will lift, how fast it needs to roll, what batteries you will be using, etc.
4. One microcontroller can run all the motors, plus whatever sensors you will need. You will need separate motor controllers for each motor, though. (A k a "H-bridges")
5. You need one motor per side, so that you can steer by driving one track faster than the other.