The one looks *mostly* complete... The only way to actually tell is to do a run-down of all the parts you have against what they were known to have, such as what is shown on Patrick's Blood and Gore:
https://www.supertool.com/StanleyBG/stan6.htm
I don't see a "cam" stop, for instance, or the longer beading stop and depth gauge (though I might just be overlooking them). These, Patrick says, are some of the most common missing parts - which he shows in this photo:
You do at least have the slitter and its stop. The often missing parts aren't used that much, which is probably a big reason they are so often missing.
I see mostly surface rust that should clean up easily.
The later models have a knob for minor adjustments on the fence that help with the plane's usability (#54 in the diagram above).
They are finicky things, however, and have trouble in all but the straightest grain. They work great in clear softwoods such as pine or hemlock. I've had difficulty with hardwoods with interlocking or wandering grain. Softer, straight grain hardwoods usually work OK (e.g. mahogany, walnut) so long as the run is clear.
A full set of cutters runs a bit high these days. If you just are just doing dados, etc., then the more common ones are pretty easy to come by.