Welcome to Just Answer. I will do my best to fully answer your question(s).
My opinion of the non-salt unit is not going to work well in your situation because you have at least a small amount of iron present. Seeing as the existing softner apparently removed iron when initially installed I am assuming the iron level in the water is low and managable with a normal softner and no additional iron removal equipment is needed. That said a sears unit will work fine and should last at least 5 years even 7-15. I do recommend having the water tested though for hardness and iron. If the iron level is higher than 3 ppm (parts per million) then do not buy a sears unit. there are other softners with different media resin inside of the unit to handle the higher iron levels.