My 9 year old daughter (we started when she was 8, attention span of a gnat), won't sew on a vintage machine because she thinks they go too fast. Obviously, depends on the kid. Mine is scared of a single ant, so there you have it.
Availability: machines come and go here. Sometimes they are absolutely everywhere, sometimes I can look for a week before I see one. I have much better luck with thrift stores that pick up donations (places that buy the amvets/vietnam vets stuff, catholic thrift stores, etc) then goodwill. You might end up with a table machine but it's easy enough to move the table along.