to sum up my knowledge about the AI (which may not be complete):
the AI learns by doing laps. skill levels, Cars and tracks are separated, so you have to train each AI driver with each car/track/skill level combination from the start. the longer they run, the faster they get. they even learn to drive the oval, which the absolutely can't with no training at all
the AI doesn't like to race each other. with single car races at the same training level and with the same setups, the AI doesn't change position after the first completed laps, they just drive behind each other
the AI doesn't pit, so they're likely to run out of fuel if the race distance is longer than the fuel in the tank
you can assign skins to the AI by creating skins in LfS that have the same name as the AI. if your first AI is called AI1, just create a new skin in the LfS garage, calling it AI1 and assign the wanted skinfile to it - if you add the AI1 driver to the field, he will use the AI1 paintjob
you can assign setups to AI drivers by using 'AI uses player setup' in the options. that way, each Ai driver uses the setup you chose at the time of adding him to the field. you can change your setup after he is added and it stays the same for the AI driver