Before buying remote buddy, I would like to know if I could use it for the following setup. 
I am using a mac mini as a spotify music server, headless, connected to my stereo. Obviously I am using an i device for controlling spotify and can also use some remote applications to sleep, wake or shut down the mac mini. But it would be very easy if I could just do these actions via the IR remote control: sleep and shut down. Is that possible?

As a bonus, it would be fine if also the standard spotify interactions are possible (play/pause/prev/next/...)

Thanks for confirming

Kris 

Yes, you can assign actions for sleep and shutdown to any button on the Apple Remote.

To hardwire these actions to buttons on your remote (so these buttons do this and nothing else, regardless of the app used), go to Remote Buddy Prefs > Mapping > Global mapping and select "Miscellaneous > Put computer to sleep" and "Miscellaneous > Shutdown computer" respectively.

When Spotify is the active application, you can also control Spotify. To customize what actions the remaining buttons do, take a look at Remote Buddy Prefs > Mapping > Spotify and make changes if needed.

FYI: you can download a free trial version at https://www.iospirit.com/products/remotebuddy/download/ and test Remote Buddy for 30 days before making any purchase.

Cheers, 
Felix