Hey mate, there are a bunch of installers and sellers in Perth. I bought my head unit from these guys.

Car Stereo, Car Audio, Car Audio Accessories & Entertainment - Perth

Price matched for me from repco, service was all right. I did the install and I have no experience with Perth installers. I have done 3 of my own cars in the past and the best results always come from split channel audio speakers "splits". You can get decent punch without a sub, though an under seat unit can boost the bass without giving you a reduction in boot space. Audio is pretty personal, go to some shops and have a listen to some speakers and see what you like. Got a budget in mind?