Owning a car means keeping up with the boring stuff before it turns into an emergency. Front brakes take most of the stopping load, and when the pads wear down to bare metal, you are looking at scored rotors and a much longer stopping distance in the snow. Fall is usually a good time to get this done before the winter rush hits the repair shops.
You should always negotiate the repair price with the mechanic before you talk about how you are paying. Ask for a breakdown of parts and labor, and see if they can match a local competitor's quote. Once you have a fair price locked down, you can figure out whether you want to pay cash or spread the cost out.
Remember that the intro window is a deadline and not free money. If you still carry a balance when those 18 months are up, the remaining amount will start accruing interest at the standard rate.