I actually have a hardware raid controller for my archive, because back in 2010, on Server 2008, the Windows software raid-5 kept getting out of sync at every reboot. Never figured out what the source of the issue was, even after a lot of testing and consulting with mainboard, raid controller and software vendors. Proper hardware raid is very fast, very reliable and very expensive. Today, I know there are numerous software solutions for raid setups, so getting a board with plenty of SATA connectors and cheap HDDs should do it. I would probably give Windows Storage Pools a try, too. I don't know how good they work, but they were designed to be more flexible than raid.
Most importantly though, however the details of your setup might turn out to be, test the hard drives and the raid controller or similar software extensively. Make sure the hard disks are healthy by using full surface scans and some benchmarks, and deliberately destroy raid arrays by corrupting a disk or removing it to see if it's able to repair.
Another thing: Mainboards with many SATA connectors usually have an additional chipset for some of those. From my experience, these don't always work as good as the ones on the Intel controller. You could also try to find a cheap SATA controller card without raid capabilities, but those are rare. I had one once, and the drivers weren't that good either.
I recommend this forum, especially the showoff thread, for inspiration:
http://hardforum.com/forumdisplay.php?f=29Edit: Don't buy an actual NAS, if you don't need all the fancy features they bring. It's cheaper to build one yourself and you can still choose to use some NAS software, or any other OS.