Given that we are now pretty much on the same page that it will be somewhere around 3000 sessions, you can remove the big box, the DL580 G8 and just add 2 more DL380p Gen8 so,
3 x HP DL380p Gen8 - 2 x XEON Proc, 128GB of RAM , 3 X 128GB SSD, RAID 1 w/Online Spare, Dual 6GB SAS HBA's for Redundancy. (Again more RAM than you probably need for 3000 active sessions but will give you room to grow)
2 x HP DL380p Gen8 - 2 x XEON Proc, 128GB of RAM, 8 X 128GB SSD, RAID 6, probably more RAM than needed but is is fairly cheap these days, or at least for the HPs at around $75.00 US for 16GB stick, at least our contract is yours might differ.
1 x HP P2000 SAS MSA w/ 12 450GB 6GB DualPort SAS Drives , you can upgrade to SSD drives. Not the ISCSI version, Direct Attached will get you the fastest speeds.
Based on Howard's recommendation,
Memcached Servers for MUC
2 x HP DL360p Gen8 - 2 x XEON Proc, 64GB of RAM , 3 X 128GB SSD, RAID 1 w/Online Spare (Again, Redundancy)
No backup solution, you can look into your own preferred solution. With a site this large you will need to do site backups, not individual course backups. So something with CDP for Moodledata ( Continuous Data Protection ).
Also, I mention HP, it is not that I say get these, I have been using HP hardware for over 10 years now so I know their hardware best and what they can handle.
You should be able to get an idea now of what you would need. Don't forget about the load balancers!