A high-end tablet is a long-term investment for your design career. When you buy the right model, it should last you at least four or five years before you need an upgrade.
Check the release cycles for the tablet you want. Often, the previous year's model performs just as well for design work but costs significantly less once the new version hits the shelves.
That 15-month window is a deadline, not free money. If you still have a balance on the card once the 15 months are up, the bank will start charging interest on whatever is left.