Where's the maths, I can only see arithmetic?

You should include the moment of inertia of the pulleys in the calculation since especially with a large reduction it will contribute significantly to the overall torque requirements.
It looks like the SY57STH76-3008B motor from Zapp is appropriate since at 300rpm the torque output will be about 1.3Nm on 50V drivers. However considering that motor is more expensive than the 3Nm motors at CNC4YOU and they will develop 1.3Nm at 1300rpm on 70V enabling you to either speed up or increase resolution I know which I'd go for...
40mm per revolution is 0.2mm/step - not a good starting point for a 'tight plasma cutting tolerance' of 0.1mm.

What diameter and length is this unsupported rail on the Y-axis?