A solid mesh router system should last you at least five years before you need to think about replacing it. Don't chase the newest release, as last year's top-tier model usually performs just as well for half the price once the hype dies down.
You want to buy for the long haul so you aren't stuck with hardware that slows down your whole house in a year or two. Aim for a system that supports the newest standards to ensure your devices stay connected without constant buffering.
That 0% window is a deadline, not free money. If you have a remaining balance after the 15 months are up, you will start paying interest on whatever is left, which makes the router much more expensive than the sticker price.