Rated RPM, rated torque, voltage and rated current would be the specs I'd look in a motor used for my robot.
If you decide about the diameter of your wheels, you will be able to get the speed in miles/h from the RPM.
And from the torque you can roughly say how much weight it can carry including the chassis and components.
For example a tracked robot with six 24v, 90mm (diameter) sized DC motor, with 130RPM, 44 kgf.cm will be able to carry 500lbs of weight (chassis and components included).
I gave an example of a heavy duty kind of robot.
Good luck.