A high-end microphone and interface should be a one-time purchase that lasts a decade. Focus on gear that isn't just a trend, as pro-level audio hardware doesn't go obsolete nearly as fast as cameras or laptops.
Watch for manufacturer rebates when new versions drop, usually in the spring or late autumn. Last year's model often sounds identical but costs significantly less once the new marketing hype settles.
The 18-month window is a deadline, not free money. If you have a remaining balance after that time, you will start paying interest on the original amount, so stay on track with your monthly payments.