Rep is basically this games version of experience, you gain rep in each race for your position, challenges that you complete and the amount of flashback you don't use. The amount given for the challenges varies based on your car, the cars unlock as you level up and the higher ranking ones generally give you more rep for completed challenges. You shouldn't need to worry about this too much if you are going for everything else including getting 1st in every event. The most significant rep comes from just winning races as you progress and the challenges with high level cars so those should be your priority. The flashback rep is negligible really so I wouldn't worry too much about that, use them when you need to if it means guaranteeing the win. You don't really need to worry about this trophy, it will come. You will get it long before finishing off "Into the DiRT" and "Platinum Performance". You will also get a lot of rep playing online going for the "Super Star" trophy.