You'r right. The door is pretty easy to be done with extrusion and I'll go for displacement on the other parts. Just another thing to learn, which is good, right ?? thanks for the reply mate!I think what they do is break it into parts. For the door, select the faces you want and extract.
There is a tutorial on YouTube where a guy makes a mini.
Displacement maps are easy.
Graph your material in hypershade - select the very first node, and plug a file in its disp slot.
For any other bits you should just be specific so people could answer you better.
That's my 2cents.
Sircharles is right,detach faces to make the car panels which you can then create the gaps naturally and detail seperately.
Some details can be done with displacement (for example writing on tyre walls) it is simply your choice as to which to model and which to displace.
From the questions you have posted it seems you are not aware that you can reinforce a smoothed edge by placing another edge close to an existing one to keep the form.