It's quite simple. For the textured materials, you only need to rename the material, for instance, instead of car00 [env:6, spec] rename it to car00. The texture is already set.
For the Matte materials, you need to assign a texture to them, as Racer doesn't support shaders without a texture. You have to create a .TGA image with just a solid color. The color is in the name of the Matte material, in ARGB format (unsure - correct me): Matte FF000000 - this is black. Alexander Knoll gave you a good tip: if you set the color of the material to a nasty pink or magenta, you'll see it on the car and decide what the real color should be.
The materials appear whitish because ZModeler adds a default reflection map - ignore it.
About the wheel: it's only one because GTA SA automatically creates copies of it in all the corners. For Racer, you'll have to move that wheel in the origin of the system, so it rotates around the X axis, and export it as a dof file, this will be the model for your right side wheels. Then rotate it 180 degrees in the top view (so it's facing left) and export again as a dof file, this will be your model for the left side wheels.