A good digital piano should feel like an acoustic instrument and last you at least ten years. If you buy a cheap keyboard now, you'll just end up spending another thousand dollars replacing it in two years when the keys start clicking.
Brands release new models every three to five years, usually right around back-to-school season or after Christmas. When a new version drops, last year's model goes on sale for hundreds less, even though the internal sound engines are practically identical.
A 0% APR window gives you breathing room, but it's a deadline, not free money. If you still have a balance after 15 months, you'll get hit with regular interest on whatever remains, so set up automatic payments for $80.00 each month.