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Rear speakers are pointless and do nothing but disrupt your sound stage, unless of course you care about your passengers that much. Do yourself a favour, get a fantastic pair of splits for the front, that will go loud enough to satisfy the whole car. VX commodores come with stock splits, have a look up near the corners of your front window, those two grilles have tweeters underneath them.
but for the price of those focals you could put R type splits in ... which are an excelent speakerI would go for this headunit JVC KW-XR616. I've seen it sell for as low as $199, so haggle. I own one, good little unit. Slightly laggy on ipod use, but I have done some research and it turns out that it's an issue that a lot of headunits have, which varies with ipod and iOS.
I would go with these splits Focal 165A1. Focal are a highly respected brand, make sure you have a listen to them in store to make sure you like the sound they produce.
And yes, that should be a mono block amp or a 2ch amp.