A RAID array or a multi-bay enclosure is about buying peace of mind for the next five years. You want something with at least four bays so you can swap drives as they fail or get bigger. Cheap single drives are fine for backups of backups, but for active work, you need the speed and redundancy of a proper array.
Wait for the mid-year refresh cycle to grab last year's enclosure. The metal boxes don't change much year to year, but the price drops when the new interface version comes out. You can use the Citi Simplicity® Credit Card (Citi) to spread that cost out while you fill the bays.
The 0% rate is a hard deadline, not free money. If you still have a balance after 18 months, the standard interest rate kicks in on whatever is left, making your storage much more expensive.