Resources for a Mac-only office
If you are in a basic office scenario, I dont think you would need much beyond a good setup and deployment of Apple Remote Desktop, and a proper application of Google Apps for your Business. 

Apple has plenty of reading material on ARD, and most of it is actually useful, for instance, the ARD 3.1 Admin Guide talks about performing basic maintenance remotely and automatically, describes how to create reports, install applications, the whole nine yards.

On to the Google section, and there are two big pieces that I would recommend: 

Why Google Docs?


Easily share documents, though for super quick stuff I recommend iChat
track revisions
collab editing
offsite storage
get free stuff like the GAS


Why Gmail?


The insane storage
spam protection
letting google handle all the email crap (99.9% SLA aka 8 hours of downtime a year)
the centralized control of groups and shared contacts
all which sync with mail.app and your address book.


You also get a plethora of other Google products for your business like Google Groups and Google Video, but I dont think those would be that useful in the organization you mentioned.

I could go on, but it is a great small business package, and it is cheap as dirt (50$ per user per year). The biggest cost I find with Google Apps is some user adoption work (though Google makes it painless as possible).

You can even sign up for Google Apps for your domain and try out 50 users at a time, or go with the 30 day free trial of Google Apps for your business (different, better, the one you actually want), that way you don't have to make a big purchase, and still see if your users would be amicable to the change.