Creating a vehicle launch demo for a high profile client and will need to allow customization of car colors. Should I rely on built in PBR shaders or is there some 3rd party shader asset I should look into?
If you intend to demonstrate vehicles with a clean surface, I do not see why you need a specific shader for that. You can change the color parameters of the Standard Shader and this will give a great result if you know how to mount your scene properly. 
There’s a big difference between the standard shader and car paint, metallic car paint. For that you want a clear coat shader, of which Unity does not provide an example of with the current built in assets. You can get something decent by using two materials together both using the Standard shader, one using an opaque metallic shader and the other using a transparent shader. Just add both materials to one object and Unity will render the object once for each material.
There are also varying quality of car paint shaders on the asset store.
The best car paint shaders on the store don’t appear to support refelction probes are badly out of date.
The technique above is a winner though! Put your paint flake normal maps etc in the base layer then add another material slot for gloss. Simple yet very effective. Thanks @bgolus