If you scan a UPnP Server into MediaMonkey you would be able to play the files in MediaMonkey, but will only see a few of the tags and won't be able to edit the files back on the UPnP Server. This means that the best method is to scan files into the MediaMonkey Library over the UNC Path from the ReadyNAS: http://www.mediamonkey.com/wiki/index.p ... ibrary/4.0
Once this is done you can modify the files and have the changes be saved back to the files on the ReadyNAS. Furthermore you can now also create Playlists and AutoPlaylists in MediaMonkey to further enhance your playing experience.
If you're only interested in playing files from the ReadyNAS (no tag editing/no playlist creation) you could access it in the Media Servers node. No need for any scanning. You can also select the ReadyNAS UPnP Server (or any other UPnP Server) in that node and use File > Add/Rescan Files, which will add the files, but still won't allow for editing of the tags in the files.
I believe the iTunes Server on the ReadyNAS will only be of interest if you use iTunes or an iDevice, otherwise I don't think it will be useful.
As for UPnP Servers some will be able to transcode files (MediaMonkey's UPnP Media Server can) and others won't be able to. This means depending on which clients you will be using you choose your UPnP Server. MediaMonkey's UPnP Media Server can transcode (Auto-Conversion) to various formats based on various rules (same as AutoSyncs Auto-Conversion) and furthermore allows you to share the powerful criteria based AutoPlaylists. On the downside however it would require the PC running the MediaMonkey UPnP Media Server to be turned on when clients want to play content.