gdgt

If you're looking to use a NAS as form of backup you will absolutely want some type of drive redundancy even if it's Raid 0 or 1 (two disks mirroring each other). Consider the fact you're making back ups to 1 single drive, if that fails you're going to be in a bit of trouble.

Based on that it will come down to what type of drive bay you want: 1, 2, 3, etc. The more drive bays you want the higher the cost in the hardware; you don't need to use all the bays up front but you at least get room for future expansion. Also the more bays will allow for different raid configurations.

So, the next question is budget. You don't need to spill your finances but a max dollar amount greatly dictates the direction someone can give you. Without a number someone may suggest a Drobo with the network attachment but that alone could be 2x your total budget before you even add in drives.

As you'll be backing up I would say that speed is more important than amount of storage. You don't want to be sitting all day waiting for it to back up.
In terms of storage I would say 2-3x as much data as you have.
You should also have another backup, see this: www.dpbestflow.org­/backup­/backup­-overview­#321