Extra slow cars:
Go to "Driver Info", modify original, then select the driver that you want to drive slower. Go to "Rating" and set everything to 0. This can be done separately to each car.
Win easily in season mode:
In season mode, set the number of cars to "1", and you only have to beat that car (usually Steve Park's #1 car).
Custom car appearance:
Select the "Opponent Manager" option on the main menu. Click on the name of the car that you like, then go to the paint shop. Click on "Export" and enter a filename for the car decals. Go to the decals for the pit people, click "Export", and enter a different filename. Save, go to your car's paint shop, and click "Import". Find the filename entered for the car decals, click it, then select "OK". Then, go to the pit people and do the pit decals. Do a little work to change the number to your own and your car will have a customized appearance.