It may not be as bad as you think. It sounds like you have a plugged drain and the condensate is leaking out of the seams in the housing causing rust. Could be worse - if it didn't leak out there, it would be on your floorboards under your carpet. Before you try replacing the housing, (not an easy job) try cleaning the drain hole with a wire and cleaning the evaporator core as described in techinfo:
If you can clear the drain, and the cleaning gets you cool, maybe you could just patch up the housing where it is rusted.
EDIT: the poor cooling may be just low refrigerant.