How do I use Time Machine to backup a network drive

Let me clarify up-front... I DON'T want to use my NAS as my Time Machine backup drive. I want to backup my NAS to another drive using Time Machine. Here is my current configuation:

iMac 27" with 2TB HD backed up to a Firewire Drive using TM. I swap out an exact duplicate of that drive once-a-month and keep one of these drives off-site for disaster recovery.

I want to add a 3TB NAS drive for all of my media. But I'd like to backup ALL of the media on that NAS the same way that I backup the iMac, with TM -- routinely swapping out 3TB backup drives and keeping one off-site for disaster recovery.

In a perfect world, I could do this in the cloud. But transfer speeds and cost preclude me from choosing that option.

Here are my questions:

1) Can TM work on the 3TB NAS, standalone, to allow it to backup to that set of new 3TB backup drives?

2) I currently have a brand-new Hitachi 3TB HD in an older PowerPC Mac Pro. Could I fire that up and share it on the network as an NAS? I know I could run TM off of that configuration. But, I think that I would be compromising speed and performance vs. the new NAS drives out there. Am I wrong? Also, I'm not sure if that configuration would allow me to stream reliably as a UPnP server.

I know it's a long-winded request, but I would really appreciate the insight of others who have possibly solved this problem. Thanks!