This looks like what you are looking for: http://www.mopidy.com/
Also I have been able to sucessfully control the desktop Spotify App using KDE Connect. This is a bit of an overkill but something to note.
Unfortunately it doesn't look like we will be getting a jukebox unless we decide to write our own. If we can make it I can provide a server to host it on.
The original jukebox basically downloaded tracks from spotify and streamed it to a webpage control embedded in the game. I don't have the time, but if I were to write one I'd use Mopidy, Icecast, Sound Manager 2, and some custom application to route commands between TF2 and Mopidy.
In that case you can give mopidy (which works with ncmpcpp) a shot,works pretty well and it got great documentation and a bunch of extensions for streaming from your google music,soundcloud, and spotify. I have only tried the soundcloud extension from the aur and it worked fine,you could even use the authentication token of your account to stream the tracks you have liked,the sets you have created and the artists you're following.
You can search and browse both local and Spotify songs. Spotify music is shown as a library, and you can add your local music to this library as well. You can also browse your local files by folder and file name. I usually browse local files and use my Spotify playlists. When I want to hear a specific song I search it.
The library will create many entries for the same artist, one for Spotify, one for local files. You can browse just the local files using file browsing (mentioned above).
A haven't had any performance issues and I have my Mopidy installed on an older laptop (Celeron CPU, 1GB RAM). How it in detail compare to MPD I don't know.
You can use any MPD client with Mopidy, so how it all works also depends on the client.
Hope I could give some answers!
Check out the Mopidy website: http://www.mopidy.com/
Actually, when I researched buying MP3s from Amazon on a Linux machine (more than one year ago?) I remember reading that the proprietary download client deletes the downloaded .amz file by default (after the MP3s have been downloaded) and that it is not possible to download the .amz-file again from Amazon.
clamz does not delete the .amz-file (which seems to be a compressed .XML-file IIRC) so in theory you should be able to download the MP3's again using clamz and the same .amz.
While my purchase worked and clamz downloaded all the MP3s for me flawlessly I have not bought any MP3s from Amazon since. I opted to instead go for a Spotify account and buying my music on Ubuntu One Music (having a Spotify account, I have not been buying MP3s for a while now though).
I am still not sure if that is the best way to go in regards to digital music but I certainly do enjoy the Spotify service in spite of its proprietariness (I am using mopidy though which is Open Source and just uses Spotify's proprietary library).