You want a desktop that handles every new game on high settings for at least four or five years. Buying top-tier parts right as they drop means you pay a premium for being first, so watch for the release cycle.
Wait until the latest generation hits the shelves and the previous year's high-end parts drop in price. This gives you better performance per dollar while the tech is still current enough to stay relevant for a long time.
That 18-month window is a deadline, not free money. If you have a remaining balance after the intro window ends, the standard interest rate will apply to the original amount, which will get expensive very quickly.