You are shopping for a setup built to survive millions of keypresses and fast-twitch gaming sessions, which means you want to buy it once and not replace it in six months. A solid mechanical board and responsive mouse will last years if you pick the right switches and sensor tech.
Wait for the late November holiday discounts to actually grab this bundle for around $250. When the new versions drop right before the sales, last year's model usually gets marked down even though it has almost the exact same performance.
That 0% window is a hard deadline, not free money, and if any of the balance is still there when the 18 months are up, you will get hit with interest charges on the whole original amount.