I've been looking into NAS boxes myself for my job. My colleague and I eventually landed on Synology's DiskStation DS420j for my purposes. However, I cannot yet recommend it, because this is still in the planning stage, and we're also running under the theory that we will be doing minor changes such as upgrading the RAM and so on.
However, this being said, if we didn't have the labor budget constraints to consider (humans are expensive), we would probably wind up going with a microATX case and motherboard and rolling our own.