For just a mac mini music server, I would recommend the default option. The hardware between the different models are the same but perhaps a faster processor and more RAM. If all you want to do is run a music server, the default is plenty.
For drives, check out this post that I made a while back.
http://www.audiocircle.com/index.php?topic=78352.msg742852#msg742852I personally feel that solid-state drives are still too costly for my library so I haven't really given them much thought. But I also use an airport express and apple TV to push the music to a 2-channel and HT setups and keep the mac mini in a different room, and so I have little need for SSDs. If I had everything in one room, then I would be looking.
Keep in mind that if you do go with the mac mini, get two drives. One for the Tunes and another for a backup. You can also look into offsite backups that run about $5/month for unlimited storage.
Since I don't use the Mac Mini to play the music but serve or stream the music to other devices, I just use what is supplied (iTunes). There's not much I would recommend that you disable, but perhaps the Dashboard (widgets). I actually run Dashboard so I can use "iStat Pro" to monitor CPU load and network utilization.
