Differential (RS-422) vs current mode signaling in noisy environments
Why do you need rs485? Using fiber optic ethernet is reasonably cheap and easy these days. This solves your problem in an instant.
As for the stepper drives, shielded cable routed away from the plasma/welder cables should be adequate.
I suspect you may be overthinking the problem. For the most part you need to concern yourself with earthing, routing and shielding.