Gentoo would actually be quite okay for what you're after since you seem to have ran that - there's some excellent resources on the Genoo site about configuring samba and integrating ClamAV for automatic virus scanning of documents. That said, CentOS would also be a good choice too
With regards to mapping drives, since they're all Windows machines it would probably make sense to add users through Samba, and have them logging in across the domain. This way it's much easier to setup their My Documents to point to the Samba server, and also allows you to easily create + control group shares if you wanted to create shared areas. They can also then login to any computer on the network without needing a local account. Is always best to think about expandability for the future
Backups should easily taken care of with a decent tape drive. It can be a fiddly under Linux though. Maybe a simple RAID-1 setup would be okay without going for something more complicated so that in the event of a disk failure, at least there's a second disk to immeadiately fall back. You should have enough storage space with 20Gb/user at those levels.