New Server Computer?

I am going to try to use a computer as a server computer to start up an old website. I think one of those G3 Blue&White towers will do pretty well. Do I even need a separate computer? I'm looking for the cheapest possible one to get. And if it is, do I need a monitor once it's set up? And, should I use the original OS or get something different? I don't think it'll get a lot of traffic, but some. Thanks!