You definitely want to run your airline in the trench with the electrical. The trenching will cost much more than the pipe.
You want black "poly pipe" for your application. Generally, it only costs a little bit more to get thicker pipe. Even though you will not be operating at high pressures, it may be worth it for the durability to move up from pipe rated for 100 psi to pipe rated for 160 psi.
This type of pipe does NOT need to be in conduit. I don't think(?) even the gravel in the driveway would be a problem. However, if you are worried, you could pour a couple of sacks of sand in the driveway crossing trench after you have installed the electrical and air lines.
