johnwasser:
You don't need precise control over position so a stepper would probably be overkill. The easiest way might be a hobby servo. It's geared down to provide high torque and has a built-in motor driver and position feedback so you don't need a separate motor controller.If you can measure the torque needed to turn the knob you can use that information to get a strong-enough servo. I'd start by doubling the required torque for plenty of margin.
I need to be able to operate the knob by hand as well as with the Arduino. Would it break the servo when operating by hand if i used a series of gears to attach it?