Buy cars that originally D class, and upgrade them some different class(A, S1, B and so on. This helps for doing in "Online Road-trip").
I did with
Class A: 1969 Nissan Fairlady Z 432(with FM Body Preset)
Class S1: 1977 Holden Torana A9X(with Body Preset and some modifications)
Class B: 1945 Jeep Willys MB(with Body Preset, additional Single Turbo and All Engine Components are Maxed out)
Then, play 5 games of "Flag Rush" with them.
On starting "Online Road-trip", "Custom Adventure - Playground Games only" is recommended.
(Because it makes progress for "#Forzathon A Bad Joke" also.)