Custom forged clubs are precision tools meant to stay in your bag for five to seven years. When you buy them, you are paying for the engineering and the fit, not just the brand name.
End-of-season clearance is the best time to buy because manufacturers rotate models annually. Last year's version usually performs nearly identically to the new one but costs significantly less.
That 18-month window is a deadline, not free money. If you have any balance left after those 18 months, you will start paying interest on the remaining amount immediately.