Honestly, based on your wants, a CarPC is probably your best bet. Plus it is installed in the car as oppose to mounted somewhere.
What OBD-II functions are you looking for? And what Android software provides them? Since you would need your USB slot in your phone connected to your OBD-II port to do this, you would be able to do much aside from that on the phone at one time. Also keep in mind that phones have much less available memory and processing that a dedicated computer. Either way, good luck from a WRX owner.