q,
I've been casually following the Avic threads and thinking about the same upgrade. From my minimal research I do know there are at least a few options from Pioneer that will work with the kit. Some have built in hard drives that hold all of the navigation data as well as extra space for you to rip your music direclty on as well. This leaves your CD slop open for use. I think most install the Ipod module and just use it as the primary source for music. I think there is/was a Z2 & Z3 model with these features plus other models.
If you haven't read the thread below have a look. Hopefully some more knowledgeable folks will respond with some more specific answers. If not, you may want to post your question on the thread attached.
http://www.986forum.com/forums/showthread.php?t=15893
Good luck,
ddb