We have a 1TB Dell PowerVault 220S (NAS), and its done the job well, although it is rather loud. However, in our current expansion we added a 2TB CX200 (SAN) from Dell. The management software is very nice and the expansion options are plenty.
If you're looking at that amount of disk space, and thinking that in the future you'll need more (Our needs are roughly 1TB/10 months, so expansion was a high priority) I'd recommend spending more upfront and going with the SAN.
Our cost with installation and a dual drive LTO2 tape library was ~$57k. A bit more than you were considering, but we were in about the same position when we got the first NAS device. We chose to spend less then, and our next upgrade cycle proved to be more costly because of it.