Our system: ZCS 8.0.3 Network edition

We were recently tasked with providing Calander interoperability between our organization of ~500-600 users and our parent organization 28K users. I don't know much about how this works, but I'm wondering about the additional load that it will place on our 2 mail servers. Anybody give me some insight? I've read the "Help"
When you search for attendee's are you given the option of the Exchange server or how does it integrate?
What does Zimbra cache? Just wondering what kind of resources are are going to be devoted to provideing user info from the Exchange server.

I have read some info on setup of the Zimbra side and the Exchange (AD) side. I don't think more than 50 Zimbra users will require AD accounts.

Any help would be appreciated.