A high-end ergonomic mechanical keyboard is a long-term investment in your wrists and typing speed. If you pick a quality board with hot-swappable switches, it should last you a decade or more.
Wait for major holiday sales like Black Friday or Cyber Monday to drop the price. You can often find last year's flagship models at a steep discount, which perform just as well as the newest release.
The 0% interest period is a deadline, not a gift. If you still have a balance after 15 months, the lender will charge you interest on the remaining amount, which defeats the point of a budget plan.