I am not a doubter, but I AM a realist. The OP sounds wary of the situation in the first place, and they have reason to be IMHO. I have done rescue for many many years, well over a decade and the vast majority of those dogs were GSD's (though certainly not all). I have had a lot of very sick very malnurished dogs come through my house, and allergies are just the worst to deal with in a dog that has a lot of them.
I am not a believer that a supplement or two fixes everything, and sure it's a great offer but what happens if (when) the supplement doesn't work? OP is still stuck with the dog, now likely bonded to it, and going to be out a lot of money. I am not saying not the give the dog a chance, but I AM saying not the give the dog a chance if OP isn't prepared for all that may come with it.
Best case scenario - they get the dog, love on it, dog snuggles with the cats at night, allergies are easily managed with diet, supplements, and allergy shots. Dog is housetrained quickly in their brand new house, doesn't have any behavioral issues that the owners haven't disclosed or don't know about due to not doing anything with the dog, taking it anywhere, etc. All ends well.
Worst case scenario - dogs allergies are out of control, not very manageable without a ton of effort and lots of money down the drain very quickly trying to find the right combination of everything, dog kills or terrorizes the cats who then start peeing all over the brand new house because they are so stressed, dog has weak nerves, reactive aggression behavior, nips, jumps, pulls on the leash, jumps the fence, marks and poops any chance they let it out of their sight for a second....
Just want OP to be prepared. They know the dog has issues. So are they ready to tackle any and all of them? I worry they aren't, given the nature of the original post and follow up post. There is no sense in the yeeha crowd convincing someone to take on a dog that they aren't ready to take on, issues and all.