Every usage will have a number attached, and the numbers quickly add up. For example, from the number and size of files you expect to store on your server, you can estimate the amount of space you'll need for that usage. Of course, as with storage in a room or a house, you generally end up wishing you had more than you thought you'd need. A server may ship with a single processor, but it may be often possible to add another later for greater capacity. A device connected to a server that lets you back data up on tape.
Servers cost thousands, so they are a significant investment, worth doing some homework on.
A good source of recommendations would be the people who designed or who maintain or manage your network. Large hard drives cost more than those without much storage space. More RAM costs more than less. If you do, you will constantly be making hard choices about what files, applications and functions to dump, and you'll find yourself replacing the server sooner than you should have. A powerful computer that handles a particular task or tasks for a computer network.