I want to drive a clock mechanism with a motor. In order to get acceptable accuracy I need a motor that can deliver a certain rpm very precisely, is low on vibrations, noise and power consumption, and has a certain longevity. I don't need "production grade" accuracy, this is just for the learning experience, but I want to come as close as feasible. So far what I found out was that it should probably brushless and be able to operate at a low voltage. Still there is a staggering selection. Which other criteria should I pay attention to?
The challenge is to get two or three separate hands moving from a single fixed rpm motor using only gears.
A stepper motor typically gives you 200 steps per revolution. You could gear that to give the second hand a step per second or half-second. You can precisely control it by microcontroller (so you'll have an accurate clock) and you still have all the gearing to play with between the second, minute and hour hands.
The stepper won't be cheating. It will be a sensible solution to the problem.
Use the plate mover motor from an old microwave. These are also available as spare parts for less than $10. These are synchronous motors which rotate at a fixed fraction of the mains frequency, usually about 10rpm. All you have to add is an additional gear which does one round per hour.
If you want 'a certain rpm very precisely', then your only options for a pure motor are a stepper motor, or a BLDC run as a stepper.
You could use an ordinary motor, servoed with some sort of encoder, but that's just a more complicated way of getting to your exact rpm.
From the comments, you consider that use of a stepper motor is cheating. It is, if you present the clock as pendulum/escapement regulated. It isn't, if you want to build a mechanical time display, that's as accurate as the timebase you choose to employ.
Depending what you consider a "motor" you could consider one of the solenoid-assisted pendulums. It avoids the sense of cheating as the period is derived from the pendulum, and, as a converter of electrical into kinetic energy is still legitimately a motor.
For example, suspend a magnet on the pendulum, and pulse a solenoid coil when the magnet triggers a Hall effect sensor.