As others have said, MDF isn't really a good choice for outdoors.
A few years ago, I would have been in the "use MDO" camp, but my
experience of late has changed my mind. MDO is indeed regular plywood,
with a waterproof resin-impregnated paper bonded to one or both sides
(you can buy it either way). The surface is waterproof. However, the
plywood inside is not treated against decay. Any unprotected edges or
holes will allow moisture inside, and become trapped between the
waterproof surfaces. Unless you are VERY careful about sealing all
exposed edges and holes, moisture will find its way into the inside and
begin rotting it from the inside out. Perhaps if you are meticulous
about painting it, and you keep it covered, and don't drive through
puddles, etc., you'll be fine with MDO, but I thought you should know
that it's not without risk.
My recommendation would be to get exterior-grade plywood, or even
pressure-treated plywood. It's typically not very flat, but if you're
going to mount it to a torsion box, you should be able to get it flat
and keep it that way. Non-treated plywood will still need painting,
but it shouldn't have the same level of moisture entrapment as MDO.
Pressure-treated plywood should last many years.