1: Then you have to race them.
2: You get an F688/S F1 car. If I remember right, it has around 900 HP, and is my 2nd fastest car around Mid-Field.
3: You win a MX-5 Miata, an RX8, a Nissan Z Concept, An AM Vanquish, a Viper Race car, and a Impreza Rally Car Prototype.
4: You get the F094/S, which is my favorite F1, and is Senna's last F1 car. It also happens to be my fastest car around Mid-Field
All of this can be found in the LOOK HERE BEFORE YOU ASK sticky, which is clearly visible upon entering the GT3 forums.
Is it worth it? I wouldnt know, the farthest I have gotten is 94%. What I can tell you is that some of the last races in the game (the F1 races in particular, and some of the Enduros) are very exciting, and are worth a try.