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.