How can I replace my main water shutoff valve?
I don't see the need for a union, just one more expense. I would replace the shut off valve with a ball valve, it will give better flow. Also I would go with copper pipe, not the galvanized.

EDIT:
Normally the Electrical Breaker Panel needs to be grounded to the water main. Because you have a non-conductive PVC water main, I believe NEC code requires that there be two ground rods connected to the Breaker Panel ground-Neutral bus bar. You should check your local codes to see if your's is up to date.