A good mechanical keyboard should last you a decade if you pick one with a hot-swappable board that lets you replace parts as they wear out. Instead of buying cheap, you want a sturdy base that feels solid under your fingers.
Wait for major holiday sales to find the best deals on premium boards. If you see a new version launch, look at the previous model; it is usually cheaper and performs almost exactly the same as the new one.
The 18-month window is a deadline, not free money. If you have a balance left over after that time, you will start paying interest on the remaining amount, which will cost you more in the long run.