A high-capacity network attached storage drive is an investment in your digital safety. You want one that lasts for years, so buy for capacity and reliability today instead of settling for something you will have to replace in twelve months.
Tech manufacturers usually refresh their lines in the spring or late summer. Check prices during major retail events in July or November when last year's top-tier models often see steep discounts, giving you more storage for your dollar.
Remember that the 18-month intro window is a deadline, not free money. If you have a remaining balance after the 18 months are up, you will be charged interest on what is left over.