As far as the output of the stepper motor, I don't think a lot of torque will be necessary to move the dial for a couple of reasons. First, these dials were intended to be turned with a finger and usually one that could be attached to a pre-teen child or elderly person, not just an adult in their prime. Second, you are planning to use some sort of mechanical drive using cogs/gears and with the proper gear ratio
it can provide increased torque in exchange for slower rotation, which shouldn't be a problem since a human operated dial wouldn't be fast to begin with. Therefore the stepper motor you have is likely to be up to the task.
As far as interfacing controlling the stepper motor... With a well made shield with libraries and online tutorials available, like the one from Adafruit you are planning on using, it shouldn't be any more difficult than controlling a servo. Although the wiring and commands necessary for the two devices are will be different.