For the install, you'll have to set up a single debian server as a
tftp server, then boot the clients with PXE enabled (ability to boot from a network share). You can prepopulate an install file so that all the machines will install with the same software. Read through the link (I just linked to the tftp section, you can start at the beginning). I never got around to trying tftp booting, although I thought it would be a cool thing to learn.
As for setting up webmail, try
this link. Again, I've never done this before...
