I posted in this thread a few pages back some of my hints.
The best way to lower a car is to use the "layers"
remove the body from the wheels, then paste the body in a separate layer.
you can then lower, raise and tilt the body.
I use this same principal for chopping roofs too
as for adding stuff the layers are good for this to. you can paste new elements in there separate layers, then adjust them accordingly.
Pshop 7 is a great tool