Hi there, am building a very similar machine for similar purposes.

You might want to rethink R&P, and use ballscrews, With stone, because the jobs could be running for many hours, you need the accuracy of Ballscrews.

Also, put a lot of thought into your spindle config, RPM and KW. ( high torque at low revs ).

A water cooled spindle is an absolute must, and consider at the design stage, you want lots of cooling water available for the cutter,and as little resonance in the Y gantry as possible