Ok, so I know it's not the most powerful - what I'm going for though is a QUIET and ENERGY EFFICIENT NAS build around $300 (minus storage HDDs). I'll start out with 1x 3TB drive and go from there - I'm a good ways from filling even 3TB, let alone the 12TB it would max at. I'm using a laptop HDD for the OS to decrease heat and power usage when idle (which it will be much of the day), I know I could use an SSD, but I already have the laptop HDD from an old laptop (it's the same model I've copied below).
Oh, I should note that it doesn't include the storage drives - I'll be going for the Western Digital Green 3TBs, probably. 1x at first, then adding more - but I'm not including that in the system budget.
I miss the controller for your self-built NAS. Why not have a look at NAS cases like Synology, QNAP or Thecus that include a parity raid controller and offer iSCSI or IP-SAN capability?
You don't want a NAS without a parity raid controller, unless you want to spend large amounts on mirrored arrays.
The NAS itself is my backup - so I wasn't going to do a real-time backup of the backup (i.e., if I lose a drive in the NAS, it's not a big deal because most of the files are backup files anyhow). And any decently rated parity based raid (i.e., rated higher than the onboard RAID on the card) are over my budget.
Also, I'm not running a business off of it - mostly just sharing some media between my two HTPCs and backing up my laptop. Nothing truly mission critical (all pictures and other things I would never want to lose are regularly backed up to a 2.5" external drive that lives in a fire-proof safe).
Mainly because I want something that is always connected and higher capacity. Much of my media is stored on my primary HTPC, but that doesn't stay on (and is connected wirelessly). I have it on the 3TB external drive too, but that's not carried around with my laptop.