Hi Virgin,

Ok well first you say the machine is working so would guess you have some type of breakout board already.? If you are not sure then you need to take pictures of the control box so we can see what you have.
There's a possibility you already have the ability to control spindle speed already as many Bob's provide 0-10v Analog output or PWM spindle output.

Next thing is do you really need to set the Speed using Mach3 or will you be happy if Mach3 just turns spindle on/off. Mach doesn't need any external boards to control ON/OFF so easy to wire up.

Post pics and we'll get you sorted.