I wouldnt like to guess on that one. We had a similar machine which I think was powered by 280w groschopp motors through gearboxes. Off hand I cant remember the reduction for the gearbox but it wasnt huge. Incidently I am using NEMA 34 motors on my machine. These would probably have enough power to run the machine as long as the bearings are in good condition and it is setup correctly. Not sure about the reduction you would need for a belt drive.
How are you going to get the CMC 2.8 to run the steppers? Does the machine have seperate encoders fitted to it for positioning?

Ian