So the final livery unlocks at level 99. The way I found the best was to do the championship on episode 10 "Around the Atlantic". 1 time attack and 2 checkpoint races.
Set the AI to easy/very easy and the other settings to your liking.
During the time attack I was able to rack up between 60,000 and 75,000 bonus points, plus the win of 100,000 and during the checkpoint races I was anywhere between 75,000 and 100,000 plus the win for each.
Each time I did the championship and won it (including winning each event) I earned between 800,000 and 900,000 points. Seemed to climb about 3-4 levels each time.
It took about 15 minutes to complete each championship. This is what I found easiest, good grinding racers!