Good for you for fixing up these abandoned bikes for a good cause.
Not to sound cold-hearted, but i'd spend a little as possible to get them safely rideable.
If it's a thing that you might be doing more of, tubes and brake pads can be bought in case quantity or 'dealer packs' to save per-unit cost.

One 'advantage' to working on those 15/18-speed MTB-ish BSO's is that they built them from the same pile of parts for a long time, so mixing and matching is easier than on modern high-end stuff.
If you get a bundle of 5 of these bike, even if they're 'basket cases' you probably can find enough good components to build 2 or 3 complete bikes; you don't have to feel obligated to save them all.