I went through pretty much the same process with mine, I settled on a 330ci clubsport in estoril blue
I just don't earn enough to justify the running costs of the M3. The 330ci has plenty of pull so I'm pretty happy with it and have no regrets.
If you like your music I would consider upgrading the audio, even the hk isn't amazing.
I just had my opamps upgraded from baris on ebay and aux input added, I then fitted aftermarket speakers with everything else stock. While you're at it you should get the upgraded firmware for the speed cameras.
Nothing else really needs changing on the car, it has pretty much all the features.