I did a quick google search and it said the Z3 comes with a 30GB hard drive.
How many songs you can fit on any hard drive depends on what format you load them in. For example on my computer i have ~2000 songs. This currently takes up 9.37GB of space. The thing to note though is that all my songs are in the MP3 format. In MP3 format you can expect a 30GB hard drive to hold 7000 songs. In WAV format probably more like 3500 songs.
It might be similar but I doubt it is the same. More than likely it will be a watered down version of the Z3. I doubt it will offer rear view camera, and other capabilities found on the standard Z unit.
It's the same unit used in the 2009 F-150. While the ST nav doesn't have the backup camera, the F-150 does. Not sure what other capabilities it would be lacking. It's integration with Sync is one of the best parts though.