I can't tell what part of the country you are in, but will assume to start frost protection isn't a factor. Regardless of where you are, you need well drained soil that is compactable. Typically 12" deep of gravel or stone dust. Many states have adopted International Building codes, so I would...
A 3/4"NPT valve seems big for that size compressor? But I think the important pieces of plumbing up an air compressor system is adequate size of the main line piping and, as in both of your schematics, pulling the drops off the top of the main line. And you really can't have too many drains. I...
Since it's detached, I would consider constructing it to accommodate 8' high doors. As to width, 9' would be my minimum. I would install the doors on one of the 36' sides. Two 8'X10' doors spaced 4' apart leaving 6' on each end. Put the pedestrian door on a 24' side.