I have set up many offices that did not like the pricetag on Zimbra Network Edition (the one with the outlook connector) and instead showed them Zimbra Desktop.
It is the free client, it works great, and it does everything Outlook can do for collaboration with a few exceptions (like easily saving email to a text file).
If you are looking to replace outlook, you don't truly need the $$$ version.
Warning: If they have portable devices like blackberry or iPhone and they want to sync their calendars and contacts to that... then you will want the $$$ version of Zimbra server so you can tell that portable device that Zimbra is an Exchange server. Then you'll get calendars, contacts, etc to your phone too.
I ran in to that little caveat and found it to be a big issue for my client. They got over though.
Dual Xeon.Dell SC1425