You are about to stop scrubbing hard-water scale off every faucet and glass in the house. Once a tank-style system is in, your skin feels different, your soap lathers, and your water-using appliances stop fighting mineral buildup. The actual plumbing work usually takes just half a day.
Aim to book this in the cooler months when plumbers are less swamped and sometimes running seasonal deals. Before you commit to anyone, get at least three quotes so you know what the local going rate really is.
That 18-month window is a hard deadline, not free money. If you still have a balance left when the intro period ends, you will get hit with interest on the whole original amount.