I don't quite follow. Any computer is more than enough for the single use you just proposed... perhaps more details on your situation?
You don't need a server operating system to do what you are asking. Just hook up any computer with windows XP, vista or 7 and share the folders you want to share on the network.
Office and data files do not take much space at all, so you don't need to worry about hard drive capacity.
You don't need to worry about processing power since you are only using it as a server.
Why don't you just share the folders containing these files in one of your existing computers instead?