General 'what to know' for replacing car sound system?

the typical good brands are rockford, kenwood, alpine, infinity, sundown audio, and digital designs. If you are going for true performance it would be best to replace everything with all 0 gauge wires. Not really sure where you can go in Australia though

Car Stereo are pretty simple things. You have a Headunit (in the front of the car, in the dashboard), and Amplifier, and speakers. That's it. In a stock system, the Headunit and amplifier are combined into one unti to save space and cost, but this sacrifices sound quality. If you are looking to replace the system and get the maximum sound quality, an external amplifier is recommended. Also, if you add an external amplifier, you will need to replace the stock headunit, as the stock units are not compatible with external amplifiers. As for brands, there are many good brands for audio components on the market. My best experiences are with Polk Audio, JL Audio, Focal, JBL, Rockford Fosgate, and Pioneer. Focal is the best in my opinion. Focal's low-end speakers produce sound quality that goes WAY beyond speakers that cost four or even five times as much. As for distributors, Crutchfield is a great distributor in North America. I'm not sure if they ship internationally, but it is worth checking out.