Let’s say based on Ubuntu 12.04 LTS 32 bit
Requirements 10GB harddrive space, min 512 MB RAM according to Ubuntu website.
Simple solution, samba shares managed from desktop GUI with scheduled backups through luckybackup at http://luckybackup.sourceforge.net for copying to shadow drives (ie non shared) -> full run down at http://www.techrepublic.com/blog/doityourself-it-guy/diy-a-flexible-network-filedirectory-backup-solution/213
This for ideas but CrashPlan seems overkill when I want to have the backup drives in the same machine than the “shared” ones – http://lifehacker.com/5919558/turn-an-old-computer-into-a-networked-backup-streaming-or-torrenting-machine-with-ubuntu
Next, figure out filesystems! I think NTFS to stay safe and have compatibility all around. See https://help.ubuntu.com/community/LinuxFilesystemsExplained for reference.