We were in a similar position a few years ago. We still have some xserves running in our district but have transitioned to either running mac minis or switched to windows for mission critical. We went away from OD and implemented an AD structure for our macs strictly for authentication purposes. I have a 48 TB synology storage device that is pointed at our AD with an iscsi lun. That stores the user folders if they want files backed up. Creating that network path is identical to the mapped drive in windows. The one thing you would need to configure properly for an AD integration are the permissions.
I also agree that one Mac Pro should be able to handle 100 connections but it depends on the accounts. If they are mobile accounts we had serious issues after about 60 concurrent users (thus the reason we went to AD).
We still use Deploy Studio with Netinstall to image our machines although this is becoming more and more difficult.
From a management perspective Profile Manager is probably your best option for cheap although I have heard it comes with several problems and there certainly are better MDM options.
Because you are an educational institution you may try to convince them to look at google. It's free for edu and is where a lot of K12 is going.