Installing OS X Server with existing OSX I have a running OSX (El Capitan) system which I don’t want to harm.

I am considering buying the OSX Server app. Apart from the cost, are their any disadvantages to doing this?

I’m thinking of impacting on performance, or interfering with using the machine normally.

Thanks
Due to its UNIX nature, there isn't much difference between running a Mac without or with Server services enabled. Even on an out-of-box OS X installation there are already a lot of background services running if needed, the difference between client and server is a bit academical in a way. 

Buying (and running)! Server.app is the intended way of using OS X Server, so there isn't any disadvantage. Basically the application itself primarily is used to administer and install all the server services and configurations (some of them you could also configure already manually by editing the corresponding config file).

At the end, these services (if activated) will need some additional CPU cycles, but this shouldn't be a problem with the hardware you are using (aka the hardware able to run 10.11), at least not if you only have a handful of clients accessing the server.