When I played around with openfoam I used gmsh to make models with.
Gmsh: a three-dimensional finite element mesh generator with built-in pre- and post-processing facilities
It will allow you to import STL files and mesh/re-mesh them as well as refine the mesh. It also allows you to link to other meshing programs. It also has some post processing capabilities.
for example
http://geuz.org/gmsh/gallery/f16_stream.jpg
http://geuz.org/gmsh/gallery/f18_stream.jpg
http://geuz.org/gmsh/gallery/screenshot2.gif
As far as modeling a car you can go and get a 3ds file off the net of your car that someone else has made. Then you can import it into blender and export it out as an STL file. Then you can import it into gmsh to manipulate.
Be for warned that doing this stuff can take a lot of CPU time and memory.