A high-end tablet is a long-term investment for your creative workflow that should last you several years if you pick the right specs. Focus on screen resolution and pressure sensitivity so you do not feel the need to upgrade again in twelve months.
Tech companies refresh their lineups on predictable schedules, usually in the spring or fall. Look for last year's top-tier model during major holiday sales; you get almost identical performance for a fraction of the cost.
That 0% window is a hard deadline, not free money. If you have any balance left after 18 months, the bank will charge you interest on what remains, which can get expensive quickly.