Hi, Ron. I have had cars that went 200,000 miles and cars that would only go 40,000 miles. It's the same for your cast iron. But I think you got your money out of it. And aren't you kind of answering your own question? You are getting scattered leaks. You are going to do a lot to replace the plaster and do not want to have to redo it. Sounds to me like you really should consider replacing as much of the DWV (drain-waste-vent) system as you can get to now. At least that's my $.02. By the way, you will probably be using PVC (Poly Vinyl Chloride), not ABS (Styrene). The plastic pipe will be somewhat louder that the cast iron for running water sounds but it is impossible to tell how much. In any case, put some fiberglass insulation in the stud bays for sound deadening.