A high-performance mesh system is an investment in your home's stability. If you pick a top-tier model now, it should last you at least five years before it feels outdated.
Wait for the back-to-school sales to pull the trigger. Retailers often discount last year's flagship routers when the new models drop, which gives you the same premium hardware for much less money.
The 18-month window is a deadline, not free money. If you have a remaining balance after the 18 months are up, the bank will start charging interest on what you still owe.