sounds like fun. I would prolly setup a win2k or win2k3 server with active directory deployed. I personnally never deployed nor administrered an active directory environment, however i think you will need this to implement roaming profiles, which will retain your user's settings when you hop from workstation to workstation.
For this machine to be the gateway to the inet, you'll prolly want a second nic which will connect to your modem. I think a proxy would prolly be a better choice over any ICS.
Setting up the webserver shouldn't be too out of the ordinary since this machine will have a global ip address. You may wanna check out
www.no-ip.com for free dynamic dns.