Looks good... Try using MS Photo editor to do an overall smoothing/sharpening.
You will need to play around with it. Some times different combinations work differently on diff pics.
Like Chevyprez said, It can help a LOT on how much or how little you do. I use it to test ideas.
Be sure to save each change so if you want to go back it will be easy. When you are done you can always just delete the ones you do not like.
Color changes can be hard with MS Paint or MS Photo Editor. I find if I change to Black/White then change color it is easier. Of course back ground colors change to, but that can be cut out if you need to. Also, a photo done in a big paved lot is easier to change car color on than one with trees and grass around it.