We had a similar issue. We are a small organization, 15 workstations plus a few Mac and Linux machines. There is one Windows server, but its hardly used for anything.
We use Scalix
as an exchange replacement, and it works really well for us (especially since we can use the community edition because of the number of users). I would suggest you atleast give it a look, it has calendaring, shared folders, free busy, delegation, etc -- plus, its based on many open source tools that you might be familiar with (sendmail) and integrates with many others (spamassasin, clamav).
I was skeptical at first about it, but after giving it a test run, it really surprised me. It isn't without its bugs, but they have a great community and feedback process.