A high-end drawing tablet is an investment in your craft, so pick one that will hold up for years rather than upgrading every cycle. Look for a screen with great color accuracy and minimal lag so you aren't stuck buying a replacement when your skills improve.
Wait for holiday sales to target last year's flagship models. They usually offer identical performance to the newest version for a fraction of the price, and the hardware difference is rarely worth the premium.
That 18-month window is a deadline, not a gift. If you still have a balance left when the time is up, the interest charges will kick in on the remaining amount.