To unlock the beautiful outside trophy or achievement and to unlock the final trophy for the platinum you will need to unlock the last livery. To do this you will require level 99. Here is a guide I made to get the most XP in the quickest time.
I can get 130k xp in two different ways both in around 2 mins of time
1st by doing a race on very hard and just winning
2nd by doing the same race on medium and drifting for points and winning.
Hope this helps.