I'm not sure I understand your question. Do you want the NVR in a "central" location connected to lots of remote cameras?
The bandwith should not be an issue.
How low a bandwidth setting were you thinking of going exactly?
2 Mbps x 300 = 600 Mbps, and you are looking at a Gig WAN connection. Assuming, the receiving end of the pipe is big and reliable enough, it's hard to spec an NVR exactly because they are usually sized by total thruput in Mbps.
When you say NVR do you mean embedded Linux appliance or is a COTS server running a VMS ok?
What's the target for pricing per channel?
There are a lot more questions before this can be answered.
Pretty much any decent NVR software can accomplish what you are trying to do.
200-300 cameras is a bit much for a single machine. but you can stack a few. just balance the cams over 3-4 servers. The rest just takes some routing.
I too have been pondering a project like this but on a much smaller scale. I'm interested in the networking/router theories with this. Undisclosed 2, can you elaborate on the "how-to" for this?
Are you concerned at all with the reliability of this system and/or violating the T&C of ISP at the cameras end?