You might want to check out Lotus Foundations (formerly Nitix).
Nitix was a commercial Linux distro originally built by Net Integrations Technologies, bought by IBM last year, and now marketed as Lotus Foundations. I only hope the decision by IBM to brand it with the Lotus name isn’t a boat anchor for a very capable product.
In my past evaluations of Nitix, I found it incredibly easy to install with a feature set rivaling Microsoft’s, while being significantly easier to use and manage. Built in support for Windows clients and VPN routing for multiple locations, email services with connectors for Outlook, and many other of the features you mention.
Unfortunately, I haven’t been able to find any current pricing info and it looks like IBM is primarily selling this through it’s partners. Nitix was originally sold bundled with hardware or separately to run on your hardware. I think IBM is continuing to offer these options.
You can download a 30-day evaluation copy from the IBM Developer site (http://www.ibm.com/developerworks/downloads/ls/lfoundation/learn.html – free registration req’d) which can be easily installed under Windows using the VMware player (http://www.vmware.com/download/player/).