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Don't go with Google Aps. Think NSA Mass surveillance! Think, Google update = breaks everything else on the system (its happened before, so it will happen again), think costs (cheap now, but eventually Google will become just too expensive to use) and then think Google only proprietary formats judging by the trend of Microsoft, Apple, Google et al lately to lock you into their little system.
No I would not touch google Apps with a barge pole! And don't get me started on the pos called Google Plus - although nothing to do with this discussion.
I can't advise on a solution for you Doc as I don't really deal with that scale of clients.
If I was looking at something like that I would like at Linux Mail server to provide the mail services and mail gateway functions like spam filtering, virus etc and I would use a webcalendar website to handle the shared calendars. Although the webcalendar is a bit limited in comparison to what Exchange can do. But it depends on what your organisation's requirements are. You can get the webcalendar to do a surprising amount of stuff and it really works well with a variety of platforms such as Ipads, Outlook (read only) and so on.
That's just my twopence!
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