I am sure someone else will come along and add more to this but here goes for starters.
Where are you going to do your drawing work and where is your machine? I have one computer in the workshop which only runs the machine software and cad stuff for quick alteration of drawings while in the workshop. I mainly do all my cad work in the house where its nice and warm.
If you go for two computers it is easy enough to network them together. I had tried this via wifi but could not get a decent signal out to the workshop so I ran in a hard link and connected it to my network router in the house. All of the computers have access to my network storage drive where I keep all my cad files so after I finish them on cad I can easily access them from the cnc machine computer.
Basically this comes down to where do you want to do your cad work, in the house or in the workshop? Also if you cant get a network in place if you go down the two computers route you can always transfer the files via a usb stick, not as easy as a network but it works all the same.