I would be concerned about a therapist who wasn't in therapy. I think the job becomes hazardous without that outlet being regularly available to them.

Also it gives them a more balanced perspective about the experience, from both sides. I daresay the same could be true regarding meds, that they could better understand what's involved with the struggle on the way to finding meds that might work for a person.

What I would be concerned about is whether their conditions and/or medication experiences might give them tunnel vision about their process (e.g., they are ADHD, and think everyone else is too), but as hard as it is to find a good therapist, I wouldn't disqualify on the basis of their having conditions, but would simply ask the question: In what way do you think your own experience with mental health impacts your perspective in providing therapy? And see if they give you a thoughtful answer. I'd be suspicious of an answer like "it doesn't affect my work at all", because we're all to some degree products of our experience, and the trick is to be aware of who we are -- not to try to be robots.