You'r getting there.
I used to do digimods, but I painted them all by hand and just used photo's to get reference from. In the good old days I used acrylics, water colour and gouche, now I mainly use Photoshop.
As it's been said before try and use the brushes in Photoshop and just use the smudge tool to smudge (rather than paint). Practice with the marque tool and create masks which will be usefull when airbrushing and use as many layers as you can to build on the image.
Here's a few examples of stuff I've done just using brushes in Photoshop to show you what is achievable, I've also drawn the girl in the pics aswell, because life would be boring if all I painted was cars.
Rich