You are buying this camera to shoot sharp video and stills that will last you a decade, which means you want to get it right the first time rather than upgrading twice. Look for body-and-kit-lens bundles during the November sales to grab glass and a sensor in one go without hunting for separate deals.
Camera makers tend to drop new flagships right before the holidays, which is good news for you because last year's model gets heavily discounted even though it is still essentially the same high-end machine.
Remember that the intro window is a strict deadline and not free money, so if any of that $1500 balance is still sitting there when the 15 months run out, you will get hit with back interest on the whole original amount.