We bought a couple of inexpensive Janome's for our sewing group in our little community, and they have worked out very well. I think they were under $100. each. I know some might think teenagers will be extra hard on these machines, but I think they'll be grateful for what you're doing.
And, I think having new machines will be much better than a hodge-podge of old machines with, possibly, lost instruction books , that may need to be serviced before they can be used. Less headaches in the long run. imho