Motor torque and RPM need to match, the work being done for proper sizing.
Weight, acceleration, speed.
Stall current will be your starting current and this will drop to idle current at full speed if there is no friction, otherwise higher.
Once you choose motor power and time duration, energy required between charging, you can choose battery size which you include in initial weight. then, you choose RPM desired and actual under load with wheel circumference and RPM. Without motor torque vs RPM and Robot mass and friction load, you have to estimate when they are equal as this determines maximum RPM.
After this is selected, with battery energy in Watt-hours , you can now choose voltage of batteries to match best options for motor. Higher voltage allows for lower conduction losses in conductors and drivers.
So make a list of specs and inputs & outputs as much as possible, including processes of speed control, acceleration steering and median differential sensor for tracking , optical or magnetic or RF line.
This is how design works. Top down approach. The result is you can then test vs estimate of performance to see if design meets goals.
It can be a simple result or complex as only you choose the performance by values of the tasks for energy, velocity, weight, acceleration, and time.