CarPC based on a RaspberryPi.
The main idea was to have a modern multimedia PC in the car that does not only rely on touch input but also has physical knobs and buttons to be used while driving.

I think that using a pi is not a good idea.

A cheap tablet will be fine.
You can build you own "driver-friendly" interface using a Pi or an Arduino.
Think that a Tablet includes also interesting peripherals like camera GPS,accelerometer, compass.

Nope a tablet is not a good start. I want to use this thing as a replacement of my existing head unit with a proper dac, lte for internet, FM/dab+, Bluetooth for hands free phone calls etc. Also it should be integrated into the dash and I want physical knobs and buttons to control it.
Additionally the "pro" version would feature two additional screens in the headrests that allow passengers on the back seats to play retro multiplayer games via Bluetooth controllers or watch movies.

But yes, an accelerometer would be a great addition.

@dsteiner I have to ask...
Do you own a Pi?

I am working on a similar system. I am using multiple devices. A tablet for the screen and as the central processing unit. A microcontroller to handle the physical buttons which is connected to the tablet. OBD2 for showing odometer and stuff on the tablet. A Bluetooth a2dp device because I effing can't figure out how to enable a2dp sink in android. Working on dvb-t connection with android for FM. It began as a small project but quickly grew in size.

@shelladdicted yep.

@dsteinerq then i encourage you to do more tests to understand how much power your pi can provide.

It absolutely can't (without a huge delay) handle multiple screen,BT, games and other features you explained simultaneously.

@shelladdicted it does not have to power the other screens, those will be connected via HDMI (plus an HDMI splitter) and are on an extra power line.

@dsteiner when i said 'power' I meant "computational power"

The CPU & GPU are not enough powerful (computational) to handle multiple screens.
(Maybe they can but with a Huge delay aka LAG and low res)
** i mean can not handle 2 different video streams.
1)You: watching at navit. (or making a phone call or anything else)
2)Your passengers watching Movie

An hdmi splitter will show the same video stream on all screens.

You have only 1GB of RAM

And everything shares a USB bus so the source of the video (USB, SD, network) will share the bandwidth with anything else. And this slowdown everything.

I like the raspberry pi.
But it has some limits. Like any other device.

@shelladdicted well I know for sure that two screens with different things work - I am not sure tough how much power the software will consume. Also the "secondary screen" is a feature that might eventually come somewhen. It could also be that I just connect a second pi to the audio in of the first for audio and let the second one do all that stuff.