Your best bet to start off with is the Hard Surface Shaders Free.
As you're using Unity Free then you won't be using deferred rendering, which is OK because the HSSF shaders are all forced into forward rendering.
If you want to make your own, grab Strumpy's Shader Editor and look up some car shader materials for Mental Ray or VRay or other rendering engines, you'll get the gist of what you need to include. There are some tutorials for UDK too.
I'd check the unity store to see if you can find anything there. If not you may have to use one of shaders that come with Unity or make your own with Shaderlab. For simple car paint, I'd look for anything with cubemapped reflections with fresnel falloff, and colored phong specular with optional specular mask.
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As you're using Unity Free then you won't be using deferred rendering, which is OK because the HSSF shaders are all forced into forward rendering.
If you want to make your own, grab Strumpy's Shader Editor and look up some car shader materials for Mental Ray or VRay or other rendering engines, you'll get the gist of what you need to include. There are some tutorials for UDK too.