Nice start.
With this one:

I would straighten it up as the car is leaning slightly, try to bring back some detail in the sky (if you use Adobe Bridge as well as photoshop, the Recovery option in Camera Raw editor is good for this).
I would, when shooting, have moved the camera a fraction higher and angled it down slightly, this would lose you some of the gap under the car and make it look lower to the ground.
As for a 50mm lens, I love mine (£130 for the Sony 50mm f1.

, completely worth it. The main thing with the 50mm is that it can go to a wider aperture (the f-stop, eg f1.

which means a) you can go to faster shutter speeds in lower light conditions, but b) it gives you a narrower depth of field (see Horney's guide
here) meaning that only your car (or whatever you've focused on) is in focus. IE, it makes the background blurry and the car sharp.