dc motors to control unmanned ground vehicle
You should start by making the Power and energy budgets. I will try to simplify as much as possible and generalize.

1-Power= Torque X speed 

The torque depends on the weight and acceleration required . 

The speed is based on motor RPM , gear ratio and wheel diameter .

Now you can decide the Power rating of the motor .
After this start selecting the preferred motor type between brush-less (HUB motor for example) or brushed (wheel chair motor for example) 

2-Energy= Power X time 

How much does the robot have to operate and in which conditions , on-road or off-road , slopes ,etc...
This is required to size your battery

This tool does all the required for you :Roboshop Drive motor Sizing