Well double bubble is probably the Home Depot bubble insulation with an R3.4 or something close to that.
Without AC in the machine room, you can use individual machine covers...I'm trying to remember the thread about covered machines and some chemical packets (they were not a desicant) but in a closed space they covered exposed metal surface with some rust preventive.
Is this a metal barn, have you selected a vendor/installer...I'm wondering because I'm planning something similar....check out redstone-tech.com about a guy in Columbus, TX that built something similar with living quarters...no machines though.
Plumbing for the compressor....I suspect that 1 inch copper would be way to go....can handle the volume and should be able to handle the 150 PSI pressure. You need to have places in the runs where you can drain-off any moisture, even if using a dryer. So basically no level runs.
Any pics of the shop?