low cost proxy firewall mailserver

I am at a non-profit agency with 100 workstations on an NT network. We are having an ISDN line installed, and are seeking low cost proxy server, firewall, and mail server alternatives to Microsoft products, whose licensing fees are completely out of reach for us. I have looked at some products, such as 602 Software’s Lan Suite, Wingate, Winproxy. Sygate, and Avirt Gateway, but have been unable to find much objective information about them as they would apply to this size organization. Does anyone have experience with these products with a comparable or greater number of users, or have different recommendations, or know where objective evaluations of these types of products can be found?