If you get old equipment to go with your old cabling, ethernet is not necessarily switched. 10 and 100 can be run as a shared medium.

Re: the sibling's suggestion of wifi: If your cabling works at 100, I think the case is pretty clear for wired 100 vs wifi; consistent 100m is better than variable whatever you get. At 10m, not so clear, especially since cabling that's that old is also likely to be daisy chained, so you're looking at maybe a daisy chain of switches running at 10m. That said, cabling is often better than the spec it was tested to, and ethernet cable requirements are for long runs in dense conduit; it's worth trying 100M on old telephone wiring if it's already in the wall to see what you can get.