A solid podcast setup should last you years, not months. Don't waste your money on cheap gear that breaks or sounds thin, because you'll just end up replacing it when you get serious. Buy a bundle that includes a reputable dynamic mic and a clean interface, and you won't need to shop for hardware again for a long time.
Wait for the sales around Black Friday or Cyber Monday to catch the best prices. If a new version of an interface launches, look for the previous model; it’s usually identical in quality but often heavily discounted to clear shelf space.
The 18-month window is a deadline, not free money. If you have a remaining balance when the intro period ends, you will start paying interest on whatever is left, which defeats the purpose of the plan.