Where to start?

[Jonathan]

Making a machine to make PCBs is not difficult since although the accuracy required is high, the cutting forces are low. However for mild steel you need substantially higher rigidity, which is plausible, but the main problem will be the requirement for a much lower spindle speed for cutting steel - probably 1/10th of the speed you use for engraving the PCBs. That's asking a lot from a single spindle since the power tends to be limited at low speeds, so having two spindles which are mounted such that they are easy to swap is the easiest solution.