A solid pair of noise-canceling headphones is a long-term investment that should last you several years if you pick the right model. Focus on comfort and sound quality to ensure you won't feel the need to upgrade again in six months.
Tech companies refresh their lineups on a predictable cycle. If you wait until the newest model hits the shelves, you can often pick up last year's version at a significant discount without losing out on features you actually need.
The 18-month window is a deadline, not a gift. If you still have a balance left over when the 18 months are up, you will start paying interest on whatever remains, which makes the headphones much more expensive.