This is also a very good project :-)
It took quite some effort, but I've got a running application built on Spotify SDK now!
It is tailored for interface with IBus in my BMW E46, but as long as you've got a button interface of sorts it should work just as well with that.
I've made a wrapper for libspotify (C#), but there are a lot of pits to fall into still due to how the SDK is designed that is not very easily understood why it is the way it is.
Login info and some settings are located in an xml file.
Search will let you search for albums, artists, and tracks.
Offline will let you toggle playlist offline status. Handy to save broadband traffic, and if wireless internet coverage is not 100%.
Shortcut key to step audio quility between 96/160/329kbit is also there for that reason.
Another shortcut will open select box with all your playlists. Premium Spotify account is required.
Some tidying up remains to be done, and I also want to test it in my car properly before sharing it. If there is an interest for it.
My car installation mp3car thread "showing off project"