A brushless DC motor should look like a constant current generator, The speed of the rotor will produce a back emf that exactly cancels the applied voltage, so the speed will vary directly as that voltage. How accurately do you want to simulate one? Don't forget that in the real world manufacturing characteristics of the same types of motor will result in differences in current and speed of the order of 1-2% for given conditions. It isn't usually worth trying to be more accurate than this. Best wishes with your work!