If we need to do only Internal Collaboration using MP8, I understand that we'll need a Web-Ex Node, but would we have to purchase a WebEx contract or incur any WebEx-related monthly charges?
Yes. A WebEx Node for Cisco MCS only extends the WebEx cloud to your premises; it is literally a member of the MediaTone network. Your Cisco AM or partner should assist you in this decision process. One option in this scenario would be to purchase a WebEx contract using the port-based model. In that scenario you may purchase internal (inside the firewall that attach to the cloud via the local node) or external ports (outside the firewall participants that attach to the cloud via a WebEx data center).