I didn't do enough homework while looking at plasma cutters and now that I have one I need a dryer before I can play with it. I hate spending money on premade when I can DIY one and gain the knowledge along the journey.
I use this for sandblasting and if it keeps the air dry for that, it should work well for your plasma. I have a small water trap 5' off the compressor, then a 50' line, then this contraption. I think the filter was $60 off Ebay.