Hint: Nitrous Oxide:
To get the nitrous from the used parts shop, you will have to lose at the drag races on a day ending with "5". For example, 305,15, etc. Return to the screen for the parts shop after the drag race. It will now be 306,16,etc. There will be a question mark at the end of all the parts, after the turbos. If your car has a turbo or if it does not appear, get out then go back in and it should appear.

Hint: Easy money:
Unlock the GT40 Concept car from the Season 1 Official Races. You can buy it for $150,000 then sell it for $185,000 to $195,000. It only takes about one or two game days to sell it, for a $35,000 to $45,000 profit.