you can add more than 6 drives with the vnxe. you can add as many as you want...just in increments of 5, 6, or 8 with the VNXe....just when you add drives you put it into an existing pool not a new pool.
as far as NFS you can deploy that on VMware and utilize the de-dupe feature. just add a switch if you are concerned with throughput.
Regarding backup: the vnxe has an NDMP feature to utilize with tape or disk.
you dont want to put your primary data and backup data on the same array...or replicate it to another array for "backup"
what happens if your array goes down, now you lose both backup and primary data. also, when you replicate...if you accidently delete something, that item that was deleted will replicate to the Dr.
If you want to stick all in the family, the VNXe3100 with a DataDomain for backup...which does inline...
