good call. The dns 323 is cheap and slow but perfect for this. You probably don't even need to buy acronis as there is likely a scheduler on the 323 interface.
Another good way to go (rather than raid 1) is the buy and external drive bay. And a couple of 2tb drives. Just use windows backup or.. my personal favourite... c:\copy *.* d: (that way it doesn't make one huge file which needs to be opened to find what's in there)
Do this once in a while on one 2tb drive, then install the other and let it run again. Now you have a 'mirror' but the drives are not joined at the hip. Also, they are not tied to any proprietary device.