I'm not sure its going to really matter. Since you're insulating, there shouldn't be very much sweating from the metal. If water gets in, it will get into the insulation, and over time, rust the floor. If you put plastic down, the water will end up under the plastic, and over time, rust the floor.
I think I'd just paint it with a couple of coats, and forget about it. Most of these buses have gone 10-15 years without fully rusting through - with leaky windows, windows just plain left open in the rain, and being hosed down inside every now and then. No more water inside, no more leaks, and a few good coats of paint should do the trick.