OS X Server is probably a little more than what you need if you just want to share files between a few people and a Power Mac/Mac Pro. It will allow you to set up better permissions for users and groups but you can share files and printers with a regular OS X install.
It doesn't sound like you'll be setting up a mail/calendar/chat server, xgrid, Xsan or complicated storage systems.
As for hardware, you're going to need a router to connect everything together via ethernet cables. You'll want a 10/100/1000 router and cables. I am no networking expert but it's pretty easy if all your staff are using Macs.
Don't forget a backup system. Check out the Drobo...
http://www.drobo.com/