These speakers are an investment in your ears, so buy a pair that will last you a decade rather than settling for budget gear you will replace in two years. Look for build quality and flat frequency response to ensure you are hearing your mix exactly as it is.
Check the release cycle before you pull the trigger. Manufacturers often drop new versions in the fall, which is when last year's top-tier model gets a price cut. Waiting for that rebate window can save you enough to pay for your stands and cables.
That 0% window is a deadline, not a gift. If you have a remaining balance after the 18 months are up, the bank will start charging interest on whatever is left, which will quickly negate any savings you made from the rebate.