I'm sure HJ can give you an informed answer on AZ requirements as he lives in Phoenix. In most areas, the homeowner is responsible for everything after the water meter. Replacing the line is probably an excellent ideas as the galvanized pipe will fail sooner or later as it very well be as old as the house. While is it is a fairly straight forward job, there are requirement on how underground connections are made. You may find that you would be wise to hire a plumber, but you should save some of the expense by doing the trenching yourself. Paying a plumber's rate to dig a 12" deep ditch doesn't make sense to me. I am not sold on PEX as a DIY job. In fact, I'm not really sold on PEX. Too many reports of rodents gnawing on it, problems with making the connections, and failure of certain brand of connectors are a few of the ???? I have. If access is not too much problem, I would stay with copper. It still the piping of choice IMHO.