A high-end mic and boom arm should last you years, so don't settle for cheap gear you will replace in six months. Buy quality once and you won't need to shop for audio equipment again for a long time.
Check the release cycles for your target brand. Manufacturers often drop new models in the fall, which means last year’s gear hits the clearance racks in late summer. You get the same professional sound for much less.
Remember that the 18-month intro window is a hard deadline. If you have a balance left when that time expires, you will start paying interest on the remaining amount, which will quickly cost you more than the savings you found.