Once the box is hacked then you can use any 2.5" PATA disk as long as you stick to the 5400RPM rule.
If you are intent on sticking with the FM modulator route then Kenwood did release an update to the KCA-R70FM... appropriately labelled the KCA-R71FM. You're alternative, is to replace your OEM stereo with a Kenwood one that supports disc text of which you can either buy a brand new one and pair it with a KCA-S220a or look for one in the same series as the one listed in my signature which is known to work with the PhatBox.
The latest hack tools are here:
http://forum.phathack.com/index.php/topic,1641.0.html, although are about to be replaced by a newer version shortly.