A high-resolution graphics card is the engine of your editing workflow. You want to buy a card that will handle large files and heavy processing for at least three to four years so you aren't stuck upgrading again next season.
Tech companies release new models like clockwork. Wait for the newest generation to drop, then watch for the previous model to go on sale. That is when you get the best value for your money.
That 15-month intro window is a deadline, not free money. If you still have a balance once those 15 months end, you will start paying interest on whatever remains at the standard rate.