a) The RAID is for redundancy. The backups are rolling. I am a backup fetishist. If I could get the department to pay, I'd have them going to a server in another country, too, as I do at home. Don't worry.
b - d) This server is used by maybe 2 people at a time, a couple hours a day, on weekdays. I think we have about 10 courses on it. It is a 2011 21" iMac with an i5 and 8GB RAM. It is a tiny server. And it performs much, much better than the virtual server we used to have through the university.
This is not a Moodle server for an entire campus. This is the Moodle server for an English language department, and most of the teachers don't use it.
Again, I don't have a performance issue (huh-huh); I have a backups-not-finishing-because-they-are-huge issue. I suspect that the best way around this is to host the files elsewhere on the same server.