A good fitness watch should last you three to four years. You want something with solid heart rate tracking and GPS that doesn't quit. Don't waste money on a budget tracker that loses its charge after a year. Buy the one that actually helps you train.
Most brands release new hardware in the fall. This means last year's top-tier model becomes the bargain choice during holiday sales. It has almost all the same features but costs way less than the newest version on the shelf.
The intro window is a firm deadline. If you have any balance left after those 15 months, you will start paying the standard interest rate on it. It is not free money; it is just a way to delay the cost.