Welcome to the forums.
The Outlook Connector is a feature of the Network Edition of Zimbra and that's a paid-for version, it's not available in the Open Source version although you can connect to a ZCS server with Outlook via IMAP or use the
Zimbra Desktop.
If you are keeping the Windows server then Zimbra can use that as an external authentication for the users but you will still need to create the user account in Zimbra. There are a couple of scripts in the forums that will create the users from the AD server but with only 25 users it's probably just as quick to manually create them. There's also a migration wizard in Zimbra that will create the Zimbra users from the AD server but it is the initial release and still needs some work on it.
You will also need to keep the passwords in ZCS an AD synchronised manually (there's an RFE to make this automatic) but again, there's a script in the forums to do it on a regular basis.
The supported platforms are the ones on the Download page for ZCS, if you're going with the NE version then I'd recommend RHEL and for the Open Source version I'd suggest CentOS (it's a binary compatible rebuild of RHEL and free

) - operating system choice is always a hot topic in Linux and I'm sure you'll get more advice about that. You will need a 64bit hardware as Zimbra will only be available for 64bit systems after the current ZCS 7 version.