Hey.
So as the subj says, how do you know what to choose?
In this instance, the engineering part already exists; even a place to mount a gear. But other than knowing I need a motor w/encoder that can report direction and speed (I can use another sensor for absolute positioning data), I don’t know how to measure for it.
How complicated is this?
Can I apply a torque wrench or a strain gauge and measure the force required, add in a leeway/efficiency buffer say 50%, and buy a motor/geartrain? Does it need to be xxx% overspec if your rated duty cycle on a H-bridge or PWM driver circuit will be less than full speed? Or whatever Et cetera.
I love experimenting, but motors/geartrains are expensive, while I’d love to have crate after crate of them, it’s not feasible right now. Per application, I want to buy 2 - primary & spare. That’s it. 
So knowing how to do this is important. Many sites cover this, from handicap assistance to combat robots to process automation. But the guy who sells you the motors seems to me the best guy to ask!
Thanks for any replies.