The (first) question here is not, “What server do I get?”, but, “Do I need a server?”. Most people are just looking for some way of centrally storing files and the simplest way to do that these days is by using a NAS (Network Attached Storage) box. There are a few different options from companies like Netgear and QNAP supporting all kinds of features that you may (or may not) need. Check out the features you need and look for something that can be backed-up to an external drive.
Your other options (depending on your needs and resources), would be to look at Linux (I personally prefer Fedora over some of the new distributions like Ubantu) or Windows Small Business Server (for home offices you could even look at Windows Home Server)