This project must have an unrealistically low budget, otherwise there are quite a few Enterprise solutions that will do all OR a combination of these tasks.
> how do you present it back to the clients?
Look at a NAS, not a SAN. ie NetApp or 3Par C series.
> And how do you back it up?
Disaster Recovery replication to another system or hosted services. NetApp, EMC, 3Par, etc, etc
> Many SAN solutions have a maximum volume limit of only 16TB
NetApp Infinite volumes limit is 20PB
You can contact a sales person from any of those companies to answer any of these questions.