More goes into this decision than you've presented here, unfortunately.
First of all, doing some backup tests from your clients should at least tell you how fast they can pitch data. You want something that can keep up as best as possible. This may vary depending on what backup technologies you're using, since that can impact the decision.
Secondly, whether or not you can take advantage of deduplication will also impact this decision, and that tends to be backup-technology dependent. Something that can to dedup will have fundamentally different demands on storage than something that just copies everything every time it runs.
Third, retention periods greatly impact how much storage you need to retain. If you really do need to keep 7 years of backups for this, that's a very different game than just keeping 7 days worth for disaster-recovery purposes only.
This kind of storage doesn't have to be the fastest stuff on the market. 7.2K RPM drives are pretty much made for these kinds of demands, they're cheap and big. Just keep in mind you do want a few disks in your setup regardless of space requirements just so you can commit data fast enough. There are many NAS devices out there that support more than two disks inside them.