Yeah your pretty much on track to what I'm imagining. I thinking using a centrafuse front end running on a 4th gen barebones "car pc" (from www.mo-co-so.com) is def the best solution to drive this thing. I'll get the solid state HDD, GPS reciever, FM transmitter and wifi card locally so I'm not blowing a lot of USD.
As for the monitor replacement, Def gotta find something with a form factor small enough to cram into the dash. The Double DIN Lilliput 629 or 701 are both are 7inch (800x480 res), so it might be a tight fit. I also found a 6.4 (600x480 res) from http://www.cybertouch.com/s6401x.html. This smaller screen might buy the space required to fit it within the OE housing, problem is it seems these only have one input either serial or USB and not both.
What I'll really need to figure out is how to incase these in the OE screen housing or pull out the old drummel to make the hole a little bigger. That'll be a pain as then I'll need to mod the whole sliding action. This is something that is a probably a deal breaker.
The button idea is feasable, technically not an issue. but again I really am trying to keep the full OE menus working. (trip counter, fuel usage meter, etc.) I'd only replace the buttons that are not part of this. Im looking to installing a mic as part of the hands free integration and then all that would be needed was to assign voice commands to certain centrafuge actions. Now I'll I gotta do is make it some like kit like these guys: http://www.knightridergps.com
But we are in aggreement, its a no brainer. this does everything except change the OE menus to english. But this isn't a big deal as I also imported my girlfriend from japan.