Personally, I think you are using the wrong tool for the job. The subjects do not care if you shoot film or digital. Maybe to other film users it is impressive. To someone who knows nothing about film, they don't care. They just want some photos of themselves taken. What you should have done is to rent a cheap DSLR like the 40D and some mid price zoom lens like the Sigma or Tamron 24-70 range. And also bring your Pentax with you. You would shoot both film and digital. Then, later, you can decide what you want to use. If the clients want color, you give them color. You don't explain to them why BW is better. You can shoot BW in addition to color.