Galvanized pipe will begin to corrode as soon as it is place in service. Copper is by far superior to galvanized. It doesn't rust and doesn't build up inside. I would not use PVC, but that is personal opinion. Don't confuse pressure with volume. You do need pressure to push water through the pipe, but a small pipe diameter with high pressure will not carry the volume of water of a larger diameter pipe with lower pressure. It's the old math formula of Pi x Radius squared. If you are irrigating from this service, besure the meter is large enough to carry the volume needed.