Lucasssssss
·Oh dear, that's a mistreated one. But the crown alone may be worth what you paid. Please take it to watchmaker inmediately, that heavy corrosion outside does not bode well for the inside.
I think that caseback is not actually corroded just thick with grime, it looks to have been on a nato like strap and not often cleaned. I would bet that this would clean up really well if taken to a watchmaker, I think the movement will be fine.