You want a desk that lasts a decade, not one that wobbles after six months. Buy a solid wood top now so you aren't shopping for a replacement in a few years.
Wait for the August back-to-school sales when home office brands drop their prices. Check if there is an outgoing model from last year available, as you can often snag the same quality for a lot less.
Remember that the 18-month window is a deadline, not free money. If you have a balance left when that time is up, the bank will start charging you interest on what you still owe.