For ShopBot and Mach all you should need is the software, comm cables and a controller card. Card wont care if there is no CNC attached. Maybe add a BO board to supply power and add a few LED's. This would get you actual "at the machine" simulation.
If you are looking to simulate your designs into finished products, nothing will beat Vectric softwares previews. Download a free trial and give it a go.
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