Considering Office Communications Server 2007 R2

I've been asked to deploy OCS 2007 R2 and I'm looking for a little advice on the installation.  I don't know much about the product and I am just beginning to research.  I'm aware of the miles of detailed technical documentation available out there, however, I would like to get the perspective of someone who has deployed it in a similar environment and what I would need to get it up running.

We have about 100 users and growing steadily.   Here are my questions:

1. Our current environment is Active Directory,  Server 2003.  Exchange 2003.  We will be looking to upgrade our environment to server 2008, SP 2010 and Exchange 2010 this year.  Should we upgrade those first or does it matter as far as OCS is concerned?

2. I know this product can be used for Instant Messaging throughout the office. What are the other main features that will benefit us knowing that we use SharePoint and Outlook heavily?

3.  For the amount of users we have, should I deploy Standard or Enterprise?  What is the main difference?

4. Does OCS need to be installed on the Exchange server?  On the domain controller?  Or does it need to be on it's own server?  Is a VM ok?  Can it run on a server that is serving another purpose or is it best for it to have it's own server?

Those are my initial questions. I appreciate the feedback..