I have never used a sang board, but I can tell you that grbl would be a decent choice. It runs on an uno and you can just jump to external drivers for a heavier duty machine like I assume you would have for your plasma cutter.
The real challenge I think is getting good CAM software for the application. Plasma can't be run like a printer or a mill from heat I understand (note that I am not an expert). For instance, you need to pierce fully before you begin cutting in the xy plane. There are also some climb vs conventional requirements that you must satisfy as well.