Good headphones are an investment in your sanity on long flights or loud trains. If you pick a reputable pair now, they should easily last you three to five years before the battery or ear pads need a real overhaul.
Tech trade shows like CES in January usually trigger new releases. When the new model hits the shelves, last year's version often drops by a hundred dollars or more, which is the exact moment you want to strike for the best value.
The 18-month window is a hard deadline. If you do not pay off the full balance before that time is up, you will start paying standard interest on whatever is left, which defeats the purpose of the deal.