Isn't おもしろさそうです a typo?
I think さ is added only after いい(よさ) and negatives (なさ) with そう?
I thought it could be the "noun version" of おもしろい, i.e., おもしろさ + そうだ (hearsay), but I think there would have to be だ before it - おもしろさだそうだ. And I don't think that would really fit here.
If anyone could confirm, that would be great.