For a booster at the start of a race, push forward at the start of the countdown, then just as 'Go!' appears, press A, and you'll rocket off!
On the Title Screen or Race Start Screen, press L five times, R five times, L five times, R five times, then press L+R+Z. If done correctly "Free Money" should appear on the screen. Although it is 500,000 credits it should be enough to buy what you want.
Complete the game on career mode to unlock Pion 6 in arcade.
To unlock Team Starcom, simply play XG Career mode all the way through to the end, winning the race on Pion 6.