The Combat Rating of your car is calculated using the chassis type of your car, and the weapons' offensive capabilities plus armor defensive abilities on board.
Question: (Jant) Is the amount or type of npc cars that you get dependant at all on what value or quantity of items you are carrying?
Answer: (Sam) It can sometimes have an effect on NPC CR, yes, if you're carrying high value items. It depends too on the talkativeness of your client.. how likely are bad guys to have overheard your deal etc.
The actual vehicles the NPCs have is somewhat affected by your vehicles, but not by your cargo.