A proper mechanical keyboard is a tool you touch for thousands of hours. If you buy a cheap plastic board with soldered switches, you will end up replacing it in two years. Spending around $200 on a sturdy deck with hot-swappable switches and solid keycaps means buying once for the next decade.
Hardware companies refresh their lineups every fall, which makes back-to-school sales the sweet spot. Retailers clear out last year's flagship stock to make room for minor updates, so you get identical build quality for substantially less cash.
Keep in mind that 18 months of zero interest is an exact deadline, not free money. If you have not paid off the full $200 balance before that promotional window closes, standard interest rates kick in on whatever balance is left over. Set your monthly payment to clear the balance early so you never owe a cent in fees.