The previous comments are fine. I would just add a couple of points:(posted 9705 days ago)The lighting is very important, whether you use a "proper" setup or daylight. Put the original on a easel, or something, the camera on a tripod, filling the frame with the original. The light should be nice and strong, to illuminate the whites of the original, but not to reflect off the shiny black parts.
Life is much easier if you can beg/borrow/steal a baseboard with two or four lamps, and a rack-and-pinion column to raise and lower the camera. While we re at it, a macro lens for the camera would be a good idea.
This sounds like a community project, or at least non-profit. In this case, you may find a friendly shop will lend you a second-hand setup.
If any pictures are partially yellow, a yellow filter over the lens might help, so the stain doesn't show in the final print. A picture that is yellow all over doesn't need this.
Automatic labs are probably a bad idea, especially a chromogenic film like XP2. As someone said, a hand-printer can easily increase the contrast of a print, but only within certain limits.
Good luck!