Gerry is right about the top RPM, but that can be compensated with a different TPI lead screw. If your lead screw is hard to change (and it probably is about impossible considering that it's a mill) then you do need to consider that as another limiting factor. Not sure why 50 IPM is so horrible? Seems plenty to me... especially for a small mill.

And yes, lots of people use switching supplies. And they do regulate, but switcher are special and can handle it where other types of regulated supplies may not. I was concerned that he might purchase a non-switching regulated power supply.