Hey Paul,
since the Onefinity CNC is a stand-alone CNC that already includes a computer, you don’t need a computer to run it.
However to model 3D in a CAD software, I would rather dedicate a computer to do 3D and graphics work in general, with very big monitors. For this, the old truth applies more than ever: There is no substitute for a lot of RAM than a lot of RAM. Or, in other words, you will hate every computer when it starts to swap memory to the swap partition.
Also I recommend a wheel trackball on the left hand (as a right-hander), to have the right hand free for keyboard shortcuts. Navigating through menus is a total time eater, always learn the keyboard shortcuts of every 3D modeling or graphics software.