First lets look at a fundamental question. "what is a server".
a server is a machine that hosts a software service.
yes you can get fancy rack mounted servers, and you can get kits to take those rack mounted servers and add feet to them and put them on their side, the word server typically conjures up images of very high spec machines, running in rows of racks in a data centre...
but rewind a little here.
you already have a workstation running windows, all versions of windows can share files, which is all your basic network file server is going to do.
All versions of windows can run apache, which is a web servers
All versions of windows can run MySQL (a database server)
All versions of windows can run FTP servers for external file access (and there are plenty of free ones).
why not just play with some software on your desktop machine first and get an idea of what you need and what you actually use before you spend any money.
(also, with no idea of what a server is, or what you can do with it or what you want to do with it, how did you decide on a budget?!)