On the long term the best scenario is to replace all what shows signs of any damage. Worst is the spalling (surface material coming loose) on ring and pinion. Other parts show a reasonable amout of indents and scuffing. Rust on gears doesn't need to be a problem as long as you can easily wipe it off with something like Scotch brite, but if not, the rust ate itself too deep in to the material, and might cause more severe damage on the long term.
Go for the best solution you can affort...
FWIW, transmissions, though of a somewhat different kind is my job
