No, spaying her later by a few months will not impact negatively.
What can be dangerous is leaving a year or two, when your cat might then be spraying AND in the habit of spraying. Sometimes then even when the hormones are removed (by the spay) the cat still might spray. Mostly though, they stop... I've spayed and neutered cats that were fully functioning breeding cats for years and have not had one continue to spray.
I agree with WitchyWoman that you really need to make sure your kitten is healthy before such an invasive procedure. For a neuter, maybe...as that is not nearly as invasive as a spay is...it's major abdominal surgery!
You mention that the chronic diarrhea is only now sorted out, what was the issue and what did you do to resolve it?