First is what type of OS is the more reliable on a NAS?
-> well … it depends … i would rather ask which raid-levels are more reliable :=)
-> Windows-Clusters nowadays seem to be reliable, clustered fileservers in a linux cluster which internally use a cluster file system are also very good, NetApp is doing clustered NAS-services….
what is a NAS Filer?
=> an appliance by Network Appliances which serves File Serving [NAS] purposes; proprietary but widely distributed, they have a specialized file system and OS
How are group permissions and permissions in general handled?
=> depends upon the Network File System Layer used [SMB/CIFS or NFS are the most widely distributed ones]
hope i could help :=)