Sorry for the late answer. I'm sure any standard 20*4 LCD works with my code. At first my OLED didn't work properly, so I made some timing change to better follow the datasheet. Later it turned out, that the problem was with the supply voltage instead.
I'm really happy with the audio quality of the Discovery. In one of my projects I use an external I2S board with a Discovery, because I need audio input too. The onboard codec have way less noise than the external one. I think STMicro made a really good job with the F4Discovery board.
Thank You Thierry, I use this box design for my other small projects too. The wooden sides are made from bamboo cutting board.