I am really impressed with the Synology package, my existing box has been excellent.
You should give
XPnology a look then. It's the official Synology software, which seems to be open source, so you can use it on any system. For free. That way it's very easy (and cheap) to make a great NAS with excellent software.
The other option is indeed (as Mark states)
unRAID. I've been using it for several years now and it never failed me. The nice part is, if only one disk fails, it can rebuild. If more fail, the array is toast, but you can still pull the data of every independent disk that isn't broken. So nothing is ever completely lost. (unless your system is struck by lightning or something).
So you think they will be fine used as normal drives then?
I'm no expert, but I would think yes.