Best backup solution for a small business with 3 pcs NAS?

I have a friend that has a small business who asked for a backup solution. There are no servers and his primary concern are his local files and quickbooks. He is also concerened about security and doesn't like the cloud backup solution too much. I was leaning towards an NAS and having systems use windows backup. Any thoughts? Is NAS the way to go in this case and if so any recommendations for good reliable NAS storage devices?