its not too hard

hardware wise you need car audio bracket/mount, 2DIN , on the RPi you will need a DAC Amp hat like this

https://www.seeedstudio.com/JustBoom-Amp-HA...-Pi-p-2846.html

If you have Car amp already then you just get a cheaper DAC hat without amp. If you dont have amp you cant drive car speakers.

For RPi you use RuneAudio , if you want to play games then dual boot with Retropi.

The problem is with the single HDMI slot unless you have the official 7" display. If you do the HDMI can pull to roof monitor. If not then you forgo the option.

Make sure your Hat support 12V stepdown ya. Also make sure you have on/off switch, otherwise it drain ur battery since you dont connect to IGN / ACC wire.

also make sure you pull out the USB cable , after install it's impossible to access them . If you are installing into a car with USB port like Almera the connector is different (car Head unit usually use USB B connector from behind ) so u need converter.