>>3100435No film or digital camera is really invisible, unless it's a tiny spy camera. Best you can do is to carry something that looks quaint and nonthreatening. SP35 or QL17 should be fine, given their 35mm FL, as long as the camera is in good working order. But those were consumer cameras, so don't expect them to hold up to abuse.
Check out the Canon P/Canon 7. They're affordable, solid, and should be easy to find in good working order. Get a Canon or Voigtlander 35mm LTM lens to go with it. The 35mm framelines are hard to see on the P if you wear glasses, though.