upper respiratory infection?

I was supposed to adopt a cat but i was called and told that she had a respiratory infection. She was given antibiotics and I was told she was out of isolation and I could come and get acquainted with her before her surgery (spay). Then, I got another call to say that she was sick again, supposedly with an upper respiratory infection again. I don't know who is diagnosing her but I'm afraid that she will get diarrhea from too many antibiotics and that she doesn't have an upper respiratory infection but something else.
She did test negative for Feline Leukemia but I don't know what her actual symptoms are, so I don't know how the diagnosis is being made, especially since this shelter does not have a staff veterinarian.
I'm worried that i will never get this cat and that she may end up being euthanized because there is no one qualified to make an accurate diagnosis or give her the right treatment.
What can I do?

This kind of thing is very common with cats in shelters.  They may not have a staff vet but I am sure that they have vets that they can call about a sick cat.  My cat's vet does this for two rescues here in this area.  People who work in rescues also get very good at taking care of routine illnesses and knowing when something isn't routine.

You would rather get her all healthy and spayed rather than have to deal with an illness right off the bat.

I was able to go over to see her today and finish the adoption paperwork. They seem to think she's well enough to be spayed.However, while I was there, she sneezed and a significant amount of infected-looking mucus spewed out. I've read about a lot of cats who never really get well after leaving the shelter. Hopefully, my own vet will be able to get a better handle on what's going on. I can't wait to get her out of there.

Awww, I hope you are able to get her out of there soon.  Poor little thing.

Would it help if you signed something stating that you're adopting the cat and leaving the shelter with it, taking her to the vet for veterinary care, and then when her respiratory system can take it getting her spayed? If all of that was on your dime?

Because the 2 boys were 8 months and 1+ years old, my vet neutered them immediately and my then 8 month female was spayed I think a week later. All 3 if I waited on the free spay/neuter were going to wait a month. The shelter gave me paperwork to have the vet sign saying they were spayed/neutered as well as a copy of the bill. The shelter was happy that I had them checked out and worked on and I was glad to offer up my 3 free spots to someone else who needed it.

Just throwing it out there in hopes that something I said helps with this situation.

She's scheduled for tomorrow but she'll be examined by the vet first. Since she still appears to be sick, I don't know if she'll end up having the surgery or not. In that case, I'm not sure what would happen because I've asked a couple of times and they won't allow me to take her to the vet on my own even if I pay for it. Now that I signed the papers, I don't know if she would be released to me or back to.the shelter if she isn't well enough to be fixed.

I'll certainly keep her and you in my thoughts.

She's finally home. As it turned out, she was already spayed. The vet found the scar when she was prepped for surgery. That was good because she came home and started getting adjusted right away.I appreciate all the feedback and suggestions.

Such great news!  Glad to hear she is finally home.