Generally, you get all the level one stuff before you get the level 2 stuff.
I actually kept my Honda Civic SI through the entire game. I had all the unique upgrades for it so it ended up being almost as fast as the WRX that I made....only it handled much better.
You really can't go trading in cars during this game because if you start to fix up a car and then trade it in....it's not easy to make the money back.
You can pretty easily supe up 3-4 cars...maybe a 5th. But you can't afford to start making a car and then sell it.
IMO unique Turbo is the best thing to get in the game.