Are you planning on using basic Windows Server, or the Windows Storage Server?

A different approach worth considering would be Storage Server, utilize the Storage Spaces. This gets you redundancy just from a dumb JBOD enclosure. Skipping something like a Drobo.

I do see NUC units out there with Thunderbolt, and I believe I've seen JBOD enclosures for it. Or you could use a Thunderbolt to eSATA adapter to use the common JBOD eSATA enclosures.