A high-capacity network attached storage server keeps all your files in one place and backs them up automatically so you only buy your storage once. If you get the right hardware, it should last you a decade without needing an upgrade.
Late autumn sales are the sweet spot for tech gear because manufacturers usually drop new models around then, meaning last year's proven versions get marked down hard. You get the exact same performance for a lot less cash.
That 18-month window is a hard deadline, not free money, so make sure you set up auto-pay so the balance is gone before the promo period ends and standard interest kicks in.