For me, low power-draw is also a concern, since either a NAS/fileserver, or an HTPC will pretty much run 24/7.
The Red's are actually lower power-draw than the WD Greens, both at idle and under read/write situations, so that's great. Around 3W per drive average.
For what you're describing though (unraid), consider the i3-3220T over the 3225. Newegg does NOT sell it, but Amazon does (129.99). Yes, it's only Intel Graphics 2500 vs the 4000 in the 3225, but for unRAID, obviously you don't give two smacks about graphics. The 3220T is a 35W part, vs 55W. Your system will run cooler, your fans will spin slower/more quietly, and you'll be sucking down less power when the CPU is under full load.
As far as OS choice goes... I was actually considering using Windows 8 as my OS, and having the initial drives (two Reds) be in a hardware RAID. My plan for expansion would be to create a storage pool using Windows Storage Spaces, add the existing hardware raid into the pool, then tack on extra disks into the pool. This affords me a way to expand the initial array without having to rebuild it, while still getting the initial performance benefits of a hardware-based RAID (not stellar though...as it'd be the motherboard's integrated RAID).
However, best option in this case would be RAID 1, which is sort of a waste of space, mirroring 3TB of data. Parity is probably the most efficient way to protect our data instead.
So, you could use Windows Storage Spaces alone, and create a storage pool. But this has me wondering about the portability of the array. Can I plug those disks into another Windows 8 instance, or if I had to reinstall my OS for some reason? I don't know, however this IS a feature of "FlexRAID" which runs as a Windows application... so this is probably where I'm going to end up.
Based on the bays available inside the Elite 120, I was thinking I can use up to 3x 3.5" drives, then Newegg sells this little guy you could put in the 5.25" bay:
This would allow me to continue sliding in new drives, this time 2.5", and continue to grow my pool. Otherwise, the Elite 120 isn't the best case as far as adding to your storage...