the best way to do this (in my opinion)...
firstly, back up all the 30 machines to the central server. have this run as a log off script in your domain so that the end users press shutdown and their work is automatically backed up to the central storage...
(you want your logoff script to do a differential copy so that it only copies things that are new, or the copy on the local machine is newer than the copy on the central server).
this will let them have a local copy of their work that they can access speedily, (assuming that a centrally stored network copy won't be sufficient).
after this you should backup your central storage to a tape drive...
(and this is where it get's really (really) expensive)...
LTO 5 tapes can store 1.6TB nativly, and almost double that when compressed
http://en.wikipedia.org/wiki/Linear_Tape-Open
if you can wait till LTO 6 then that's 3.2TB native capacity and you won't need to use hardware or software compression and your backups will happen faster... and you'll only need 10 tapes, instead of the 30 tapes that you'd currently need....
you'd also need a tape drive.
and a robotic loader...
and the backup software to control each of these...
also to back up 30 TB in an evening, you'd need something of a miracle...
a speed of LTO5 is 180MB/second...
180MB/second = 10.8GB/Minute = 648GB/hour = 15.5TB / day.
so it's going to take you 2 full days to backup your 30TB... (so that's nightly backups out the window!).
in my experience, it's difficult to get anywhere close to the max write speeds of the media...
so you end up with multiple tape drives inside the tape loaders...
you're talking thousands of dollars per LTO drive.
thousands of dollars for a tape loader robot.
the tapes themselves aren't cheap and you need at least 30 for a full backup, if not more to have a decent media rotation...