See below. A NAS being network attached storage is different than building a file server. A NAS quite literally would be a box with limited functionality hosting storage to a network.
That'll cost you the better part of 2 grand minimum for a good one. A good LSI card with a couple SAS ports that can do 8 drives is like 300. If you take whatever you have laying around and toss it in a case with lots of bays (be it 3U/4U or even a full tower) you have expandability instead of needing to purchase yet another premade box for another 2+ grand. NAS boxes are seriously overpriced.
THis is the card I got with a promo code:
http://www.newegg.com/Product/Produ...6118230&cm_re=lsi_raid-_-16-118-228-_-Product
They make the 6Gb/s version cheaper.
http://www.newegg.com/Product/Produ...6118104&cm_re=lsi_raid-_-16-118-104-_-Product
That plus whatever you got laying around vs buying a box then needing to buy drives and having limited expandability.
There are other options for cards too, but that supports RAID 6 likes you want (although dual parity isn't really needed).