The reaction of the marble is going to add calcium compounds (hardness). I don't know the math to predict how much the hardness will be increased. You may want to add a softener, so I would at least leave a spot where the softener would go if it turns out you that will want one.
I understand that the bladder pressure tank will be replaced with a hydro pneumatic water tank, but there will be no chlorine injection or settling tank. I don't know how effective that is going to be with the iron. A softener can deal with some residual iron. Some premium resin deals with some iron a little better.
If the local guy is experienced with successfully handling the local water, that is great. But maybe you will want your system to a higher standard.
These comments based on reading and not experience.