I am looking for some advice on motor/motor controller?
At the voltage level mentioned and the power level that seems likely, you will likely find a larger selection of DC motors and controllers at a lower price compared to AC motors.

A potentiometer connected as a variable resistor is a possibility, but you may have a difficult time finding a suitable resistance and power rating to work with the motor. Also the speed stability and repeatability of setting could be a problem.

The best alternative will probably be a DC motor with an electronic speed controller. You will need to search to find out what is available with the required power and speed range.

If you want to pay the price, you can probably find an industrial product with extremely accurate speed control and a built-in speed display. Something like that would connect to utility AC power.