You need to have a script which stores the selection made. It might be easier to do if you have this script in Autoload (project settings/AutoLoad).
Make a scene which has 2 buttons and sprite or model for the car. Add a script to scene root and connect signals from both buttons into it. The script should react to buttons by changing the car's texture or model. At the same time, it should set the selected car into the first script (the AutoLoaded script).
When your game starts, have the game check whatever was saved in the first script and load the corresponding car.
Sorry if the answer is not too detailed, but the question is too broad to handle in great detail. If you run into specific problems in implementing the system, feel free to make extra questions :)