You are buying a desk to fix your back and survive long days at home, so you want something sturdy enough to last a decade. Buying a solid frame and motor once beats replacing a wobbly cheap desk in two years.
New Year sales are a good time to look because office gear gets heavily discounted right after the holidays. If you wait just a bit, manufacturers often drop prices on last year's models to clear out warehouse space for newer versions.
Remember that the 18-month 0% window is a hard deadline, not free money, and if any balance is left when it ends, you will get hit with interest on the remaining amount.