Help needed with mock-up of car

Am making a mock-up of a car with polystyrene for a theatrical road show. Have finished the basic form but an a little lost on a light yet durable and paintable finish.

Also is there a lightweight foam that I can use to cast out the tires.

The entire car needs to be really light as it has to be flown in onto the stage.

Any and all help will be greatly appreciated.