How long do you plan on living in the home? Til the kids are grown?
1.5 cubic to 2 cubic single should do very well..
Your local dealer most likely has more years in that area than most of us here in treating your water.. any number of us might have many years in water treatment but I know little to nothing of water in AZ....
If a single is bought now and 5-7 years from now you feel that a twin would be better then an upgrade most likely would be the way to go as you already have the one tank and the salt tank,, and depending on the system that you go with .. might even have the meter and bypass already plumbed in ..