This late in the game, I would recommend going with Windows 10 Pro. Windows 7 will be end-of-life January 2020.
JamesNT
Not a bad idea, but that's 3 years from now and almost a generation cycle in the computer world. By the end of support they may be ready for a new "server."
I have a few issues with Win 10, but then I had a few issues with Win 7, it's really what you're used to. That is why I would agree it's not a bad idea. Especially since you are talking the same money. This group are more likely more comfortable with 7 than 10 however.
Absolutely wired with a gigabit switch. The difference in speed is significant as well as lower footprint for hacking.