Barb,
I just picked up one of these at a local antique store. There are 4 variations of this plane in it's life cycle. Mine is the last version. The one you have is the third version.
My first attempt at cleanup would be to try WD40, 0000 steel wool and a bit of elbow grease to see if it cleans up enough to use. If not, I'd move to Evaporust or Metal Rescue to soak it in for a few hours.
I was lucky, the one I bought has no rust. All I need to do is sharpen the blade and see how it works.
TonyC