Definitely ended up with more than 13 rooms, but I figured the more ridiculous the better. I ended up making a kind of maze in the basement with gnomes at the dead ends. It actually turned out pretty funny because people visiting the museum go through the maze to view the gnomes so it's super busy down there. I can imagine people bumping into each other in the dark and getting spooked by the ghosts on their explorations.
My sim completed the mansion baron challenge 4 days before she died and I tried really hard to spend all her money before the end. This is the only challenge I have actually attempted. It was fun!
Mine is the Lockwood Mansion on the gallery.