I know noting about your mill, but the place to start is to basically work out what you have and what you can reuse.
Mach3/mach4 requires compliant hardware to control your steppers / servos. So I would suggest by having a look as your drive motors to determine if they are stepper motors or servo motors (photos + check and write down the wiring).
Then trace the wiring back to the control board, is it one board or lots of motor controllers? (Photos, wiring diagram traced out / sketched out)
Mach3, mach4 requires that the motors are controlled by step and direction signals..... hence you have to determine is your motor controllers are able to accept step and direction signals... I am guessing it's probably one control board hence you may need to source motor controllers...
Then you need to determine what other stuff you have...
Spindle control (on/off, pwm, step and direction?... photos, trace wiring out)
Coolant control
Probe inputs
Anything else? (Tool changers?)
Basically you need to trace out your existing wiring in a high level (block diagram schematic) and in a low level detailed (cores, colours etc detailed schematic)
Then you need to determine what else you want in the end if your machine does not have it (mpg, tool changer, whatever)
What you are trying to do is find your current hardware requirements, what you need to replace, and what inputs and outputs you require (can you get away with a parallel port connector of inputs and outputs, or do you need a multi port motion controller?)
A dedicated pc or laptop running mach3 or mach4 (or any other cnc software) is always better than sharing a pc or laptop.... note that mach3 requires a 32 bit version if you want to run the simple printer port (and some newer machines dispite having a printer port have displayed problems with using the parallel port).
Personal suggestion... get a motion controller.... the timing is better, machine tends to run smoother, less chance of timing glitches, more potential for newer windows operating systems than xp or w7 32bit.... future pc replacement upgrades or if you have a problem with the pc and you need to swap it out for a replacement quickly).
You basically need to get to know the innards of your machine to start with then create a shopping list of requirements before moving on to doing the conversion.
Get yourself a big bound notepad and camera and start taking notes and sketches and photos.... please feel free to post and I'm sure someone will help