This will be longest grind in game. You can check unlocking conditions for any part in customisation menu but they are basically same for every character & car. To make this short.
1. You have to win any Grand Prix with every car of every character (so it's 8x4 GP's).
2. Have 500 secs of drift, 300 nitro uses and single 5 sec drift for every car. Depending on yours skill it's like 10-12 3 lap races for each of 24 cars.
Once you have other skill depending achievements grind for this one should take around 20 hours depend on skill. On higher difficulties have higher top speed..