Get the Gallardo as soon as possible and upgrade all of its performance upgrades. Everything will be almost full. Try to get the handling better than the speed or it will have no effect. You should now be able to win all events and defeat the Bosses easily.

Get the Audi TT as soon as possible and tune it up as far as possible. Everything will be almost full. Then, buy the Gallardo as soon as possible and tune it up as much as possible. It will be one of the fastest cars available.