Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison
Here is what I run for all our lines except the press. The press is fed by copper. Good stuff and easy to install. For a hundred bucks you can add a couple of lines and make it look super clean. They sell their stuff on Amazon at a discount.http://www.rapidairproducts.com/?gclid=CjwKEAjwi9K4BRCQzq7d1c6A_XASJABueAO2vjZKTwArqE82vbY-Yld07o0gQ3M7L6Gr88lsnHH5dxoCtRDw_wcB
Same debate here, pex-al-pex or black iron. Already have rubber for the drops. Since the pipe is in the ground could you flush it with a mild caustic or something? Though you'd want to be sure it all of it was removed before attaching equipment.
Let me say one more thing about airlines. I am not an airline design expert but this is how I see it. No matter how dry your air is going into the pipe it will condensate. Sending air under ground will force condensation because of the drop in temp. Lets say its very little almost none. Even a little water under grown like that will have no outlet. So it will build. It will build to the point that your pipe will fill over time. Now you have to push your dry clean air through water to get to your press. In my opinion you should run your air up hill as far as much as possible in your system. What this does is drains the pipe back to the compressor /chiller. This way you can drain off the water.