You want a digital drawing tablet and stylus that feels like real paper and lasts for years, so you only have to buy it once. A good setup handles your daily creative work without lagging, skipping lines, or wearing down your hand after hours of drawing.
Late November is the best time to buy because of major tech sales. If you can wait just a bit longer, last year's model often drops in price while still offering almost the exact same performance as the brand new release.
Just remember that the 18 months is a hard deadline, not free money. If you still have a balance left when that window ends, you will get hit with interest on the whole amount.