The problem with most atom boards is they don't come with very many sata ports that allow expansion. So you will most likely need a board that comes with at least 4 sata ports, and a PCI or PCIe slot to add a RAID card if you want to expand beyond that.
Once you factor all that in, you're looking at a cost somewhere similar to a prebuilt NAS from Synology, QNAP or Asustor. I used to build my own NAS and use FreeNAS. But I've since switched to Synology.