What's wrong with Sophos (apart from not being free)? It's what I use. I know most if not all the free options don't run on server 2012.
Have a look at CLAMAV, I've never used it on Windows so I don't know how good it is? Seems to work OK on my RaspberryPi, if like the Linux version then you will have to write a script to interrogate the Scan Logs to see if any virus were detected
http://www.clamwin.com/
ESET File Server, no complaints here, eset has pretty much been rock steady for years.
Have they baked the installation so it pulls the updates from their work server and you can't enter update credentials yourself?