I just wanted to chime in here to give a response to your AD design question. I can see using separate OUs if you think you will need to apply different GPOs for different departments. Of course you can still manage this with one OU using security filtering, but that also becomes an admin burden at times.
I haven't decided if we will use online or cached mode for Outlook. I think online mode does make the most sense, however I do not get to make all the calls 
Mike