Well just to be more precise, I don't mean your idea is wrong. But any complex, expensive setup eventually will run out of space (even for lack of expandability). You can further expand if you distribute your storage through multiple hosts (using iSCSI). No need to have multiple samba shares, if that is an issue: you can even join drives from different hosts with mdadm or lvm, or (more safely) with aufs on the main rig (say the master node).
But any of you hypothetical "slaves" can be anything you want, even a 9u/50 bay unit.
Or, if you feel adventurous, you can take a look at gfs, glusterfs or so...
Just my two cents :-)