I want to do one of these also. Assuming you have a good spindle, screws and ways, it should be feasible even if you cannot use the existing drive motors. Some of the older servos may be a problem to interface with the newer digital electronics. I forget the exact criteria the steppers/servos need to meet, but it has to do with the signals between the motors and the board. I would consider using a motion board, vs a break out board. The advantage here is you remove the "work" of pulse counting, etc from the computor and it is done on the motion board instead of by the computors processor. When the CNC is doing complex motions, it does a better job if no interruptions happen. I have a bench top that does well enough with a breakout board and stripped down Windows PC, but I make pretty simple parts. I use Mach because my machine came that way, but would probably choose Linux if were building a system. I am not an expert, but am just passing along info I've seen in blogs or white papers over the past year I've been interested.