I have done work like this in the past and short of specialist museum archival cleaning, the best thing, considering the age and possibly less than ideal storage conditions, is to print 'as is'. There's not a lot of worthwhile advancement to be achieved in the darkroom process as a hobbyist - no disrespect intended - except for perhaps a careful wash and air dry using distilled water and a cotton swab. Try on one first that you think would clean up, and if it doesn't work leave them alone, the gelatin can be fragile or irrevocably shrink causing crinkles. Always explain to your 'client ' what you are going to try, and get their permission after having explained the potential pitfalls. I wouldn't try any further chemical enhancement.
I have a box of 1930's negatives I got out of a junk shop. I printed a few that were @2"x3" and I liked the old look, but bare in mind that most old negatives were not meant for enlargement, generally, but for contact printing, which is when they look ther best. I would go down this route for the larger negatives as they would add up to a nice sized personal album. For modest enlargements I would try some subtle sepia toning to enhance the look.
Your carrier idea sounds fine, though you will probably need some anti-newton ring glass for the upper surface - Are you charging your neighbour for this ? As for a lens an 80mm will cover @ 6x7 or a 105mm for the maximum size of your enlarger which I think is 6x9 (?)
At the end of the day you have to let your neighbour know what they can realistically expect, if it's some 'straight' printed images from unrestored neg's. then there should be no complaints. People I have done this for in the pasts have always been delighted to have images returned to them that haven't been seen in 2 or 3 generations.
Really for any measure of image restoration it would be best to turn to the digital path, it's what it can be made to be very good at afterall.
Regards, Mark Walker.