With the unit being unplugged and bypassed, I would guess it's probably too far gone to make repairing it pay off.
It depends on where your water comes from as to how hard it may be. Well water can be quite hard where lake or river water is quite soft. About 7 grains hard on average. Well water; from 3 to 45 grains hard in my experience.
You said they have an iron filter which makes me believe it's well water. There wouldn't normally be iron in lake water.
It never hurts to have your water tested, but a test for all 400 and some odd things that could be in your water is cost prohibitive. Hardness, PH and Iron are the main things in well water. Chlorine, Ammonia and Fluoride are the main things in city water. City water is normally safe in all other respects. So is well water from a good deep source.