you're asking a very broad question. plus i think you might have better luck in the armor or scratchbuilding forum. in general, dents can be made by heating up the plastic and deforming it to make dents. also you could remake the body work in brass which would behave more like actual metal bodywork. rust is painted on, you could also add some texture by gluing on some fine particles before painting.

ohh..

well i did heat up the plastic and made a few little dents plus thats how i made the cage...just heat up plastic tube and bend it where i need it to bend...what kind of stuff are you talkin about when you say glue it on befor paint?

don't know, they do sell rust pigment that'll do texture too but i just get some fine sand or silt from my driveway and use that.