motor controller

Varying the DC voltage will allow you to dial in the RPM. However, you didn't say anything about the load conditions or if it was a static load or dynamic.

If the load varies, then you must use a closed loop circuit to counteract the changes in motor load.

In real world applications DSP microcontrollers are used for this purpose and instead of varying the DC voltage the power to the motor is pulsated (Pulse Width Modulation or PWM) as a square wave. The PWM has a frequency, which is usually set to some constant frequency and the amount of time that power is applied (on-time) for each cycle of the waveform (duty cycle) is varied to control motor torque.

Brushless DC motors are the motor of choice these days versus a permanent magnet DC motor.

Now that you have been given a general direction to go, look up motor controllers, DSP, and brushless DC motors on the web and you will get a better understanding of the task at hand.

I was just wondering if this was a question or a statement?

Have you chosen a DC motor yet, or is that what you need help with?