I actually am using the Garmin Mobile PC software. Since it was discontinued, it became easier to "find" online, if you know what I mean. I use that through the Apploader plugin, coupled with an old Garmin USB GPS18 Receiver, and it seems to work pretty well. I'm not sure, but if the Garmin Mobile PC software works with a USB receiver like this:
Amazon.com: GlobalSat BU-353 USB GPS Receiver: GPS & Navigation
You could save a hell of a lot of money instead of buying a license for Sygic/Destinator.
Lilliput screens are the screens I have been looking at. I made the mistake of not researching my screen before I got it, as several people have reported erratic touch response. The plus side, it is a capacative touch screen, and the brightness is incredible.
The aux cable is pretty easy. ^.^ I was also looking at flush mount USB cables, but I haven't looked too hard into the solution for that. If you find one, let me know!
Also, it's awesome to see another CarPC in the 370z's! Hope everything works out to your plans, since I can say this cost me more than I thought it would!
EDIT: I found the website I got the AUX cable from:
FRONTX - Webstore - Pay through CCNow Inc.
Turns out they have flush mount USB's and stuff as well. It's perfect for CarPC needs.