Looks like you found a whale!. Given that amount of storage required, I would be looking at the NetApp solutions, which isn't low cost but depending on the model will have the throughput required to pull it off, and they are built more for the high availability, large scale solution you need. What VMS are you planning on putting on it? with that many cameras you may want to look into a system that does archiving so record to one set of drives then offload to a storage solution which if done right you can stagger the archiving periods to better utilize a NAS or SAN's throughput for longer term storage and lower cost drives.
Sounds like approximately 90 day retention (2GB/camera ~ 2Mb/s H.264 over 90 days) - guesses on my part. I would be going after the retention requirements first: Do all the cameras need to run 24/7, or can you do motion detection? Can the storage be stripped down over time e.g. after 1 week or 1 month, keep only 1fps from the video? How fast does older video access need to be? Can older video be off-lined to tape or other media? (Recurring cost, but less live storage tradeoff) How reliable does the archive need to be? Can the older video be stored in a non-raid configuration (e.g. Coldstore-ish with powered off disks). Can I reduce total bandwidth by reducing frame rate, reducing resolution, or increasing compression and still have an acceptable image? Depending on the simultaneous viewing requirements, it may make sense to split the storage and localize it per building, or building cluster.
You might also consider Seneca Data server/storage solutions as well.