Russian,
I have lots of experience with the discovery board.
Our car (The Flying Lumberjacks) has a digital dash backed by one.
You can't make out anything but the fuel gauge on video. http://www.youtube.com/watch?v=KwcOhuL-yRU
What we're currently monitoring is:
Battery voltage
Oil temp
Oil pressure (switch only, no actual sensor)
Water temp
Fuel level
There's lots of inputs left on the board I made. The board has seven analog ins + battery voltage montitor and 10 digital ins. the display is a 240x64 pixel graphic LCD, it's not great but you can see it in the car pretty OK. Future upgrades will include data logging to a USB thumb drive.
Looking at your schematic for the input in post #2, your pull up resistor should be going to 3.3v not 5v. Most of the inputs onthe STM32F4 are tolerant of 5V but not all of them. There should also be diode clamps to protect the CPU from voltage spikes. A unidirectional 3.3v transorb is better still.
Once you start using the A/D converters you should note that the reference voltage on the board is 3.0v, the only way to find that is to look at the schematic of the board. It took me a little bit to find that when all of my sensor inputs were off by 10%.
For something as simple as an ECU I wouldn't bother with an OS, it doesn't supply solutions for any problems you have. The ST supplied peripheral driver libraries handle setup of the hardware just fine.