500TB is nothing these days. You can easily buy any system and it will support it. Look at FreeBSD/FreeNAS with ZFS (or their commercial counterpart by iXSystems). If you want to have an extremely comfortable, commercial setup, go Nexenta or with a bit of elbow grease, use the open/free counterpart OpenIndiana (Solaris based).
You can build 2 systems (I personally have 3, 1 with SAS in Striped-Mirrors, 1 with Enterprise-SATA in RAIDZ2 and 1 with Desktop-SATA in RAIDZ2) and have ZFS snapshots every minute/hour/day replicated across the network for backups, both Nexenta and FreeNAS have that right in the GUI. The primary system also has a mirrored head node which can take over in less than 10s. As far as sharing out the data: AFP/SMB/NFS/iSCSI/WebDAV etc. whatever you need to build up on it.
My system is continuously snapshotted to it's primary backup so that in case of extreme failure (which has not happened in the 7 years since I've built this system) I can run from the primary backup until the primary has been restored with perhaps a few seconds of data loss (don't know if that's acceptable to you but in my case it's not a problem in case we do have a full meltdown)
Where are those systems limited to 16TB? I wouldn't touch them with a 10-foot pole because they're running behind (within a few years a single hard drive will surpass that limit).