I pick this, " Lastly I could keep metal , install 1/4 insulation over and then wood. Lose about 1/3 inch height".
A slight addition I would add is a vapour barrier over the metal ceiling as the first layer, maybe even reflective material, whatever that is called, Refelctrix?
Using cedar will help absorb any condensation you create in your bus if not treated. Let it colour naturally. Voila, nice ceiling and not to intrusive height wise.