A good curved ultrawide monitor replaces a messy dual-screen setup and gives you enough room to have three windows open at once. If you buy a solid model, it should easily last five to seven years of daily work and occasional gaming, meaning you are investing in your setup once instead of replacing junk in two years.
The best time to buy is late summer during the back-to-school sales, when retailers drop prices to clear out desk gear. Look at last year's model instead of the brand-new release since manufacturers usually just tweak the stand or the speakers while keeping the actual panel and curve the exact same.
That 0% window is a hard deadline, not free money, so if you still have a balance left when the 18 months are up, regular interest kicks in on whatever you owe.