I think you need to be realistic about CNCing stone - what do you plan to do with it? Not saying it cant be done but compared to a machine for cutting aluminium the rigidity and cutting forces are much higher. An acquaintance of mine has a commercial CNC for cutting/drilling/polishing granite worktops up to 3m long. The spindle on that is 10kW at low revs for use with diamond tipped cutters and the bed is a significant chunk of cast iron with a very large water reservoir to keep the cutter cool and the dust levels down.