What you need to do is find out what controls the axis motors (you mention a "step" motor, but you don't say what it is one) and what controls the spindle motor.
Mach 3 puts out step/dir pulses on two wires to each axis. These normally control driver cards e.g. Gecko, which convert the step and direction pulses into on/off signals for each phase of the motor. If this machine already uses stepper motors for the axis, then (providing they are CMOS/TTL compatible, you can probably control the driver cards directly from a computer output.
The spindle is different, in that Mach 3 can output two different signals - one is step/dir as above - but acting on the spindle controller, the other method is a PWM (pulse wave modulation) waveform which can be converted into a speed signal voltage to drive an inverter or similar, and the reversing is taken care of by outputs on the spare outputs wires (one wire is "on" for forward, one wire is "on" for reverse)
What you do not mention is what computer the client will be using, and what outputs it has. Probably the best way to go these days is with a cheap laptop, and a Smooth Stepper output board, which plugs into a USB port - and provides all the outputs/inputs for the various mach 3 functions.