Ended up going the easy route and getting a Nexus 7 to use with a dash mount. The pic below makes it look like it's blocking my view because of the zoom and height of the camera, but it isn't too bad at all, though a cupholder mount would probably be ideal. I used epoxy on the suction cup because I always have trouble with the those things unsticking - it seems to hold solidly this way, and I don't plan on moving the mount itself anytime soon. My phone's crappy camera doesn't do the screen justice, it's really quite crisp and vibrant.
I plan on using it for other non car stuff (reading mostly), but it's mainly going to serve as GPS and music storage. I was hesitant about using it for music versus the Raspberry Pi setup I originally planned, but then I found out you can use an adapter (micro USB to USB) and app to attach a mass storage device for the Nexus 7. 8GB of onboard storage is about the same as my phone, but if I can add a flash drive it's more viable for music. I'm happy with the Nexus 7 thus far, it's the first tablet I've used that feels as high quality and fluid as any of the iPads. Such a good value for $200.
Using the Torque app (plan on getting a bluetooth OBD-II scanner later):