A high-quality doorbell camera should last you five years or more if you pick the right hardware. Avoid the bargain-bin brands that stop getting security updates after a year, as those end up costing you more in replacements.
Wait for the major holiday sales to buy, as that is when the top-tier manufacturers drop prices to clear inventory. If you do not need the latest bells and whistles, last year’s flagship model is often functionally identical and much cheaper during these sales.
That 18-month window is a deadline, not free money. If you have any balance left after the time is up, you will be hit with the standard interest rate, so set up an auto-pay to finish the balance on time.