I recommend starting by looking through vendor's websites. At the end of the day if you aren't going to be fabricating your own custom motor and gearbox, you'll

I recommend starting by looking through vendor's websites. At the end of the day if you aren't going to be fabricating your own custom motor and gearbox, you'll have to pick something out of a catalogue anyway, so check what's available and for what price. That will help you get a sense of what's possible, and then you can check that against what you want to accomplish and go back and forth until you've narrowed down something for a prototype.