A solid pair of headphones should last you five years or more. You are looking for the kind of build quality that survives a daily commute and the kind of noise canceling that makes a plane engine disappear. Buying the absolute newest model usually means paying a markup for a few minor tweaks you probably do not need.
The move is to wait for the refresh cycle. When a major brand drops its latest flagship, the previous version—which was the industry leader just yesterday—gets a steep price cut. It is the most effective way to get pro-level gear for a mid-range price.
This interest-free period is a fixed deadline, not a permanent discount. If you still have a balance on the card after the 18 months are up, the standard interest rate will apply to whatever you still owe.