[Build Help] test server for a CS student

I'm trying to figure out how much RAM, and how powerful of a CPU I want. I'll be using the server as: a test bed for Linux distributions, media server, file server(NAS), occasional Gmod hosting (10 people at the most). This is more of a project just to tinker around with, and the most intensive thing this server should encounter is Gmod hosting. I'll be visiting my university's surplus store sometime this week, and I want to know what I should consider before I buy the cheapest PC they have on the shelf.