Mr. gk, can you still set up AC to use just the NAS with the SQL Express backend?
I don't think you will be able to (well), but I'm not sure.
Benjamin, I only use NAS as my Backup Device and at home as file sharing for my media (Video, Audio, Pictures).
My server has a built-in 4 drive RAID5 Array. That is where my SQL Server Data Files are stored.