We had a similar problem on a much smaller scale Geographically.
We set up VPN links between two Microsoft networks with email being homed on the server with the largest number of users. This enabled us to share between buildings in different cities using high speed internet connections.
Everything worked fine with the server software right out of the box except video conferencing, (we did not do any).
I doubt the server package (Small business server 4.5) came with it. So you would still need to buy a video conferencing product.