I have been where you've been, and let me tell you, what you are considering is a waste of money.
Moore's law applies particularly well to harddrives. Every two years you can buy a new hard drive that is twice as big, for the same price. Or pay half as much for the same storage capacity. If you stock up now, you'll spend a lot of money for something you can buy a lot cheaper in two years' time.
My advice: Buy new hard drives and replace them as you run short on space. If you run out of space inside your rack, move the contents from your oldest disc into the next, and you can sell, discard or get an external enclosure for your oldest disc.
Allocating massive storage without immediate need for it, is going to cost you a lot of money.