Not sure how much space you are going to find behind the stereo in the commodore, both the holden's and ford quite often leave very little room to work with, even for stock standard stereo swaps! A CarPC might end up needing to be under a seat, or in the boot to allow enough room.
Also, think about the temps you will get in WA. If there is no space around that board, she aint going to work from October through to March if you leave your car in the sun (Just a heads up to the US people, we get summer temps up to 46 degrees C). So plan for as much space as you can get.
Also, for the amps, let me know if you see anything cheapish, im looking at having to throw down about $250 for a 4 channel amp, and thats about the cheapest any of these local car audio places offer. Its an absolute joke. *sigh*