You didn't say what the charity plans to do with the images. If they plan to use them in newsletter type publications or in web pages, I think you aren't using the right tool for the job (it's overkill). If they plan to make and display enlargements (greater than, say, 8x10), then you are. But if a P67 is what you have, then that's what you use. You may find that your images have value beyond what the charity wants/needs.
FWIW, back when I was doing informal portraits for a music association, I started with a tripod but then decided not to use it. It had an effect on the subject that inhibited them a bit. Maybe if I was better at it, I could have overcome that. But I sense that in your situation, you don't want the subjects feeling the photo is "too big a deal".
An incident meter is handy in that you can measure the light without intruding on the subject (assuming the interior is relatively evenly lit). But you do have to consider the effects of artificial existing light on color images.
BTW, do you really need to go as far as drum scans? Would good quality flat bed scans serve their purpose?