Mach3 has surprisingly little processing requirements. What really matters is no having those things which might interrupt the parallel port.
I got my PC off the curb at heavy trash day. Needed a new power supply. It's just a few years old and was one of the cheapest computers made when it DID come out. Changed from the original on-board video to a video card I threw in there and it's fine. Well, I upgraded the processor and RAM with slightly better ones I had from my last computer that had a smoked motherboard, but you get the idea.