The brush less motor is in effect a high current stepper motor. To make it work each coil is switched in turn. This requires an electronic controller to switch at high speed. Although it is possible to make on from components it would certainly be bigger than commercial units and may well be less reliable - even more costly for a one off. Most commercial units employ a microprocessor to decode the transmitted signal and generate suitable pulses for the motor. Better to buy as they are not all that expensive for what they do.