The answer will also depend on how precisely you need to maintain the speed, and exactly what kind of DC motor you are using. The reason I say this is that if you apply a constant voltage to a good quality DC motor (low impedance windings), then the speed will be surprisingly constant regardless of load.
When you apply a certain voltage to the motor, it will run at whatever speed is necessary for the back emf to equal the power supply voltage minus a small difference. Adjusting the supply voltage will shift the operating speed higher since the motor must then spin faster to generate the cancelling back emf. So, this gives you speed control. As the load increases, the motor will simply draw more current in order to maintain the speed.
So, it may be possible to solve the problem with nothing more than a regulated power supply.