A high-capacity tablet should last you at least three to four years of daily drawing. If you buy the current top-tier model, you are paying for the latest processor, which matters more for layer count and brush lag than anything else.
Wait for the inevitable spring or autumn refresh cycles. When a new model drops, last year’s version often sees a price cut or hits the used market. Buying the previous model once you see the specs of the new one is usually the smartest way to stretch your money.
That 18-month window is a hard deadline, not a free pass. If you have a balance left over after that period, you will start paying interest on whatever is remaining, which wipes out the benefit of the plan.