A good mechanical keyboard and ergonomic mouse are tools you buy once to fix hand pain and make typing all day actually feel okay. We want gear built with hot-swappable switches and solid aluminum frames that will easily last five years or more.
Wait for the mid-summer back-to-school sales when brands drop prices to clear inventory, or look at last year's model once the new version launches. You get the exact same performance without paying the early adopter tax.
That 0% window is a hard deadline, not free money. If you still have a balance left when those 18 months are up, the interest kicks in on what you bought from day one.