A high-end mic and mixer should last you years, so don't settle for gear you'll want to replace in six months. Buying the right setup now prevents the need to upgrade your kit twice.
Check the release cycles for your specific brand. When a new version drops, retailers often clear out last year's stock at a discount. Prime Day is a great time to hunt for these bundles, but only if the gear fits your actual production needs.
That 18-month window is a deadline, not a suggestion. If you still have a balance once the time is up, you will start paying interest on whatever is left, which defeats the point of the plan.