DC motor selection criteria: number of poles
It depends on your variations of poles and torque with max RPM.

You may want to choose a multiphase reluctance motor bigger than all devices under test or  the ability to parallel more than one active load each calibrated for input vs T vs RPM.  Generally the motors with lower DCR produce more torque and fewer poles more RPM/V. 

To span a wide combination of devices under test , you would have to compare costs of having 1 huge motor that has more start and high RPM torque for braking or make it flexible with multiple smaller motor options. 

Poles are like transmission gears so you can practically test a 200Rpm vs a 6000 RPM motor with the same active load but it becomes more costly as Hp increases unless you use belts and pulleys which must be lossless 

In order to make a smooth motor tester you will want the ability to add a flywheel to reduce the dynamic pulsing of each motor  .   This means less averaging of the signal from the load cell and unexpected vibrations.