Because you are talking about a spot protected from the sun, it would likely be ok to use PVC after the valves for for the trip back down to the ground. I've seen several pictures online of installs done that way. I know it's against general practice (and codes in some areas) to have PVC above ground for the mainline, but from you you are discribing, you only want to possibly use PVC for the lateral lines (the lines AFTER the valves).
But from the point of view of protection (kids playing, weed-eater, rocks kicked by lawn mower) I feel like it would be safer all around to use copper going back down to the ground. Once underground, you could then use a copper FEMALE to transition to a male PVC or male poly addapter. Notice the specification of FEMALE on the metal side. When you go to screw the metal-to-plastic transision pieces together, you do not want the plastic part to be the female side. The reason is because the metal basically is NOT going to give any in a contest between plasic and metal. The plasic will hold much better being compressed by the metal female. If you tighten female plastic over male metal, you are very likely going to crack the plastic.