Apple sells a “Mac mini” and a “Mac mini with OS X Server.” Other than the obvious differences (ie, RAM, processor, etc) the only difference is that the high end mini comes with Server.app pre-installed.
If you don’t need the extra hard drive, you can order the mid-range Mac mini ($799) and just download Server.app from the App Store.
One more thing to note, when you setup the high end Mac mini, it won’t automatically setup the Server.app. So, if you just want to run OS X on the server Mac mini, you won’t have to do a clean install of OS X.
