: 1) having sprinkler system installed ... pvc is being used rather than galvanized ... does this seem ok?

: 2) repiping is not including replacement of all drains in house. is this typically a separate job or when does one replace the drains?

: 3) i notice that the plumbers are notching some of the 2X4s in the wall to fit the copper pipe and are putting metal plates on the ouside of the stud over the notch .. is this normal?

first question =yes
2nd question== repiping a plumbing system should be defined in your contract. It should state what part of the plumbing system is to be repiped.
3rd question= In some parts of the wall it is impossable to get a drill to do the job. so the plumber has to notch the studs. It is only for 1/2" pipe and he is showing good work practices by installing nail plates.
sounds like he is doing a fine job. Dale's Plumbing Service LMP