From my experiences the fastest way to earn money and level up is through Career with as many assists turned off as possible and with the drivatar difficulty on either Pro or Unbeatable. In the Career you should do all of each classes championships, each have 1-12 tracks but you get a big sum of money for winning the championship afterwards too. This way you are making a maximum percentage of the winning prize, all you have to do is finish in the top 3, even if you place 3rd, you get a gold medal with a lot of money and XP. Hope this helped.