If you have the resources you should break the whole bike down and grease it and clean up. Then you will know if there are any real rust issues. Looks superficial but if it's there it could be elsewhere and old bikes usually have old grease. Unless seller serviced which is highly doubtful if the freewheel looks like that.

Its part of the vintage game, they usually require a good amount of elbow grease but the ride will be better.

Good luck.