Polyglot - do you have a means of processing lengths of paper on the roll (not necessarily the whole roll at once) or do you expect the built device to output individual cut sheets, which needs a much more complicated mechanism. If you want individual sheets to size, it would be easier to cut them to length and store them in batches, perhaps using a stepper motor or digital counter and DC motor. Incorporating a mask and a guillotine means winding a bit forward after exposure to clear the mask, then cutting, then winding back to position the paper back beneath the mask, otherwise each print would have a margin of wasted white paper.
In a previous career I designed paper handling devices. The trickiest bit was interfacing every possible enlarger to the rolleasel. I ended up using a photocell and plastic fibre optic to link the lamphouse to the control. This sensed the end of the exposure and advanced the paper.