I'm so sorry you have to re-home Bella, it's never an easy decision. I am a member of a few rescues and everywhere here is right, talking to an all breed or preferably a shih tzu rescue group would be a great idea. We always do home visits and vet checks and some do lengthy phone interviews. A shih tzu rescue would be the best one in my opinion.
But to answer your question, no it is not out of line to want to visit their home, you absolutely should! If they have a fenced in yard, walk the whole perimeter to be sure there aren't any holes or spaces that Bella could get through. Check out their home for cleanliness and/or any dangers that could be a problem. If they don't have pets and do have plants, let them know that there are some plants that are very dangerous to animals. You should also do your own vet check if they currently have pets. You want to be sure they bring in their pets for routine care. Depending on Bella's personality, ask about the other pets in the home to be sure it will be a good fit. Bella's personality and likes and dislikes is really what drives the interview. If anyone has a problem with you meeting with them and checking out their home, consider it a red flag and do not let them adopt your dog.
Good luck and I wish you lived closer, I'd take her!