You have a good plan. Whether the data drives are internal or external to your MacPro server is going to be based on how much storage you need - and you will also need an identical amount of storage for a backup. Every drive gets a backup - even, or especially the operating system drive. A lot of time and labor goes into setting up the server, must be protected like any other investment.
It is going to be hard to realize the speed of SSDs on a server over a Gigabit netwrok. You might set up your OS and backup drive on regular spinning drives. Then setup a pair of RAID5 arrays, small ones, for the served data. You can do that with a single hardware RAID controller and 4 drives per RAID - 8 total drives. Be wicked fast and well protected from almost anything. And each 4 drive RAID volume can be pretty much any capacity from 3TB on up to 12TB as needed.