Steve, my 2c...
In my opinion there's no point going "top shelf" with a Cobra audio system as you lose a lot of fidelity with the open top car and all the noise that a Cobra makes. Build a system that generates volume without distortion - good two-way speakers, a mid sized amp and any decent head unit will do (less flashing lights are better). Let the amp do the work, not the head unit.
All but the very cheapest of head units meet your basic requirements - CD, mp3, USB, bluetooth. Go into JB Hifi and play around with the units mounted on the wall. Take a USB stick with all your music and test out the navigation - some head units are dreadfully slow at reading large collections over USB.
Pick the one you like and ask the salesman to match it up with an amp and speakers to suit your budget - most likely a set of 6" or 9" if you can have room. If you pick a time when they're not busy they might even let you hook up said speakers so you can test with your music (not the awful techno they usually have playing).
I bought my speaker cable online. Something like this:
25m Roll - Super High-End 99.98% Oxygen Free 12 AWG 2-Core Speaker Cable
Make sure you get the speaker polarity right. Also try to keep your power feed away from signal cables. Test the speakers in different locations. I recently moved mine from behind the seats where they sounded "OK" to the foot wells and it sounds awful now.
Good luck!