A good podcasting microphone and boom arm setup should last you five to ten years. Buying quality gear up front keeps you from buying a cheap starter mic today only to replace it with a real setup next year.
Audio equipment runs on a predictable release cycle. December bundle deals are usually the best moment to buy, as retailers mark down current top-tier models to clear room for new inventory. Last year's model will sound just as good to your listeners for a lot less cash.
Keep in mind that an intro APR window is a strict deadline, not free money. If you still carry a balance after the 15 months end, standard interest rates kick in on whatever is left, making your microphone setup much more expensive.