my shop has steel pipe with treaded fittings and so have the three professional shops that I worked in previously. I have a pipe threader so it was not very expensive and there is no worry about them failing. I know allot of shops are using pex now though but low spots will trap water otherwise there is no problem
PVC will shatter. DOn't use it.
Don't ask me how I know this.
Round these parts it's galvanized pipe only.
you should be fine with galvanized piping. just remember to run a separator at the compressor and where you plan on connecting your spray gun. your probably running 1/4" fittings so 3/8-1/2 pipe would work fine.