in the past the serve.app had the name of the OS X system, now they changed it as Server, means no connection to the OS X. you can buy it now, when you have El Captain it will work anyway without update. BUT can be that some features will not work or increase a possibility to have problems. So the best solotion is to update the Server.app, everytime there is one available.
as said before it is not expensive anymore, around $20, and deservers it completly. for the first time user, you will spend some time to configure it, understand it, and experiment it. if have the will to have a try, i would buy it right away.
if you are a beginner, i suggest you get in touch with the videos publish by this fellow ( http://www.toddolthoff.com/technology/YS10.10.html ), it help me a lot and help me to have a clean running server.app on my server.
best luck and welcome