[aerohead]

There may be some safety considerations that would favor the steel skin.The cars crash behavior may be predicated upon how that skin will crumple during an impact,absorbing kinetic energy of the collision in a way not possible with composite.Don't know.
Find a wrecked door at a body shop,cut the skin away and weigh it.See what kind of weight savings you're looking at.
144-man-hours/square-ft,@ $40/hour might be an estimate for creating the plugs/molds/trim fixtures/layups/vacuum-bagging/autoclaving,etc.to create the skins.Plus materials.
Just the skins.