You can spend a lot and get a little, or spend a little and get a little... audio systems in these cars are limited not only by the noise of the engine but mostly the noise of the wind and the limitations on where you can put speakers etc.
The best system for the money we have done is a high end set of speakers in the rear wall (and I don't mean anything you get a box store) paired with a quality amp that has some real clean power. We run that right through the headphone jack of an iPhone or iPad. It's clean and relatively stealthy.
Some customers prefer a more traditional input and control method so we mount a marine head unit in the trunk with a control unit under the dash:
