I was in a similar conundrum few months ago. Here's my take on this:
I did not change head unit to Android because those available in the market are Chinese and I've heard that their sound quality is not good. I did not go for an aftermarket head unit such as Pioneer, etc. because that would be in duplication to the OEM unit already installed, and I wanted the stock look.
So I just changed all 4 stock speakers and also added 2 more speakers (without amp, and using the OEM head unit). Makes a massive difference to sound quality.
I also wanted back camera but didn't want to install a new head unit (as mentioned above) so I needed a screen. Of course I didn't want it to stick out, as mentioned above, and also because a generic screen might turn out to be a thief's paradise. So for now I've installed a rear view mirror screen because I hardly use the rear view mirror - I only use the side mirrors as rear view mirrors. That does not stick out as much so it keeps the stock look and is not as easily evident to potential thieves.
I've also done sound deadening which makes the stock system sound even clearer.
So now I have a nice sounding system, stock look and a rear view cam. Still don't have a Android unit though, due to the aforementioned reasons.
