Forged wheels completely change how a car handles and looks, but they are a serious investment. At roughly $2200 for a quality set, you are paying for the strength and the weight savings that come with the forging process.
Wait for major holiday weekends to pull the trigger. Retailers often run site-wide discounts during these times, which can save you a few hundred dollars before you even look at your payment options.
That 18-month window is a deadline, not a pass to pay whenever you feel like it. If you still have a balance when those 18 months are up, the interest rate will jump significantly, so treat that monthly payment like a utility bill.