Wow, hang on. Take a step back here.
You are going way to far to achieve the goals you're looking for.
I'd recommend a NAS enclosure with FTP capability. Map all the computers to the drive. You can assign permissions in the software for the NAS.
For the virus scanning I'd do that on each local machine. No use moving that to a server.
For the external access I would get a nice linksys router and flash DDWRT onto it. Set up port forwarding to your NAS drive with limited permissions(read only) for outside access. Register with
http://www.dyndns.com so you have consistant access even when your ISP renews your IP address.