If running under Windows 7, Mach 3 uses its own low level driver to generate pulses from the parallel port so you only need a basic interface board.
Windows 8 changed the driver model and the people who make the software decided that it was too difficult or too expensive to make a new driver. Their solution is for you to use an external motion control board connected via USB or Ethernet. This takes the place of the low level driver software that previously ran on your multi GHz CPU.
If you are determined to use a microcontroller for something, perhaps add sensors for safety interlocks or tool changes. This can be linked to the Mach3 software via the emergency stop circuit.
I get the feeling that this an attempt to use an Arduino as that interface. to allow MACH to send out the USB signals and the Arduino to see them, then control the stepping of the motors.
I have to agree with Robin2 on this. although the Arduino can, and will be used for this, I would search for someone who has done this and see if their code is open source.
the other alternatives is to use an older PC, or at least an older windows.
you can get a dedicated PC with an old version of windows for next to nothing.