Your old unit likely had fiberglass insulation in the doors (pre-1995), not the solid foam that is now being used. The newer ones do not rust out like that. Wax is a good idea, and prevents the moisture from staying on the surface of the door, although I doubt there will be any. I say go for it.
FYI, the steel used in all new boxes is about 50% thinner than on the older ones anyhow, so they want you to protect the exterior. Makes no difference which brand you buy, either.