Hi Anthony -- I definitely need more info to give you a good usable answer. As you said you want to show horror film (and you've polling three extreme Rated R films) for a fund raiser and looking to draw all age groups from 14 to seniors, I have to tell you straight up, you're walking into very precarious territory (in the area of offending), and you have to choose very carefully.
What type of fund raiser is it? Raising funds to do what? for what organization? What's the venue? outside screen, your house, a church, a school gym? what audience you targeting?
First of all, you have to consider the law, and offending protective parents. Rated R films are "Under 17 Requires Accompanying Parent or Adult Guardian". If you do a Rated R film, how are you keeping 14-17 yr out who don't have parental approved chauffeur? You can be charged with
Contributing to the Delinquency of a Minor. And also, there are law concerning displaying these films for money. I don't know if using a film for a fund raiser is legal or not.
Again, I need more info to give you good advise. But I'd say, IF you use a Rated R movie, you should restrict the audience to 18 and up. Or you may consider great horror films that aren't Rated R, or simply don't have 'obscene' material, such as Poltergeist, or even Frankenstein 33, or Nosferatu 22. Better yet, you may want to show a movie that has great clips from many films, where you can control the content.