It depends on where you want the scratches and how 'worn' the car is supposed to be. Maybe just the fenders will do the trick. And those should be too hard to UV.
Regarding the model, it does look fairly CG-y. One thing that helps sell your model is minimizing the number of perfect corners. The back of the car has perfect corners. In the real world, it's really hard to find a perfect corner. They're almost always rounded of to a certain degree. So give them a bevel when you're nearly done. Don't do it immediately, because editing beveled geometry isn't as easy as non-beveled.
Besides that I don't think the problem is really in the model. I think it's the lighting and the shaders. Put an HDR image in there, look into some more advanced shading tutorials.
http://www.fireinthesole.com/