Money
After getting the second bus, start a tournament. Tournaments are series of races that give a reward based on your final position. You can also get regular rewards throughout the individual races, making them a good way to earn money. Start with the Championship tournament to make money. Ignore the Knockout tournament as you will lose often. Participate in single races for quick money. Try the Hills Race because it it only has two laps, and you can easily finish in the top three. Any two lap track is a good choice to earn extra money. Watch the "Extra" category because single lap races will occasionally appear there, allowing money to be earned even faster.
Upgrades
You can only upgrade your bus before a race. At first, focus Speed And Acceleration and upgrade it twice. Then upgrade Grip twice and the Boost once. Save the rest of your money for purchasing the third bus and repeat the process.