It appears to be just surface rust. Probably sat in someone's damp barn or basement for a bit. I don't see any pitting.Disassemble, Wire brush it, then soak the metal parts in phosphoric acid diluted to 20% for 4-8hrs. Then use, a urethane wheel to do final clean, then move to the normal prep you do to the plane and you got it.
I basically want to get it usable. And use it. There's something relaxing about watching a craftsman using a plane. The sound it makes.If it was in better shape I'd buy it. I am starting to gather a set of MF planes.