When doing "power levels", my preference in general is to follow the show's method: fast and loose, according to the needs of the story and setting, but with an overall "feel" in place. I should stress that Wish's special talent magic is meant to be nearly unintentional, being tied to intuition and emotion. For example, plants that she sings around every day usually look more vibrant and healthy than those she doesn't.
Because of how her magic works, she couldn't cause someone to miraculously recover from a broken bone overnight, for example; but if she came in to visit that same pony at the hospital every day, they might get well enough to be released earlier than normal - saving maybe a week or two on an expected six month stay.
As for stuff like poisons, pretty much up to the player and anyone else interested. Maybe being around her improves the pony's mood and this strengthens their immune system (some evidence for this working in real life, and Equestria being permeated by magic makes it more plausible). Or, maybe it has no effect at all.
Bottom line: her purpose is not to be powerful. Like most non-protagonist ponies in the show, her magic is incidental to who she is, being mainly a tool to help her with her occupation or hobbies, and we can tune it as is dramatically appropriate, if we think it's necessary. She does have the usual "action at a distance" ability that all unicorns possess in addition to whatever their specific magic specialty is (as far as I know).
Finally, she was born and raised in Ponyville; as for contributing to the story, here's something she could do: visit little sick foals and colts, read them stories and sing them lullabies. She'd enjoy that. I don't know what's currently happening in the thread, or the metaplot if there is one, but if some players give me a summary of some recent events / current arcs I'm sure I can find something.