You want a network-attached storage system that lasts five years or more. Don't buy a budget box that crashes in two; focus on a reputable brand with a good app for your phone. When you buy once, you save the time and headache of migrating data later.
Tech manufacturers usually refresh these boxes in the spring. If you wait for the holiday sales, you can often pick up last year's model for a steal. It will perform just as well for your photos and files without the premium price tag of the newest release.
Remember that the 0% window is a deadline, not a gift. If you have a remaining balance after the 18 months are up, the card will start charging interest on the rest, which gets expensive fast.