Ok so for you pick your part'rs, here is an example of a bad boost and a good booster. The booster with the corrosion on the lip where the M/C connects has obviously had a fluid leak and the brake fluid unfortunately ruined the booster. This booster passed all the vaccum tests I could do with a hand pump, but unfortunately, after even one millimeter of brake pedal travel, all vacuum would be lost and the brakes would go rock hard, like pushing a stack of cinderblocks with your heel.
I pulled two other boosters from the pick your part, and both had cleanly painted lower lip/rims, and both work flawlessly. A piece of advice I would give, and am currently giving, is only buy a used booster if that inner rim/lip still has clean paint and no evidence of fluid leaking in.
Here's a good picture of one of the great boosters, this one is currently installed in my W123 and is quite powerful! It is quite nicer than my original attempt with the broken booster that I sanded, painted and buffed. *sigh*
Again, a clean painted lower lip/rim where the M/C goes through indicates that the Master Cylinder (M/C) O-ring seal was still holding vacuum and keeping fluid out. A fresh and nicely painted rim is quality insurance for a good booster.
Definitely do not buy a booster with a corroded lip! It'll just be a big shelf piece!
As for the rust around where the master cylinder bolts to the face of the booster, that is likely caused by water that gets between the M/C and the Booster, but does not make it past the seal. It is perfectly harmless. Or as least as harmless as rust gets anyways.