I have supported Servers in schools, where all the Macs are wired in and any user can log in to nay computer. I use the same setup at home. In this case ALL User files are on the Serve, which may be more extreme thatn what you wnat.
Gigabit Ethernet (with jumbo frames) is a requirement to get "near-Hard-Drive" performance. Your Router need not have this capability, since most transfers are directly between the Server and the workstation, and do not involve the Router.
I use a small VelociRaptor 10,000 RPM boot drive as a System Drive on the Server, and 600GB VelociRaptor 10,000RPM Drives set up as a Mirrored RAID for User data, then Time Machine (hourly) the Users folder of the Server onto a "regular" 1TB external.
My son has started to carry a laptop, loosely linked to the Server as a Mobile home, and I warn him that if he wants backup, he needs to remember to Sync to the Server.
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.