I'm still using the same motor I used with the adafruit motor hat. A smaller nema 14. It has plenty of torque at 12 volts. However, I have purchased a 12v nema 17 motor with 0.9°/step (400 step/rev) as an optional upgrade. I'll decide what to keep once I've used it a bit more.
As for attaching it you have options that you will have to work out what is best for your case.
You can:
Direct drive the focus shaft for zero backlash, but may not have the step resolution within the CFZ (critical focus zone)
Direct drive the fine focus knob. May introduce backlash, but increases step resolution.
Gear driven focus shaft or fine focus knob. Minimal added backlash, allows for gear reduction.
Belt driven. Use fiber reinforced belts with enough tension to avoid slack when changing directions. Belts also seem to slacken in the cold.

For hardware, there are shaft couplers and motor brackets. But truly the best asset is access to a 3d printer to produce custom gears, mounts and enclosure.
For connections I use DB9 sockets that can be screwed on.

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