As much as I love vintage machines (i have 3) If I were to buy a machine for my young adult nieces at this point, I would buy a entry level basic machine from Walmart or a sewing outlet. That way, they will have an owner's manual to consult, most come with several different feet, they would have a warranty *some better that others, and most of all, it would have zig zag and some other stitches built in.
Yes, the new ones are not as good as the old workhorse pre 1970 Singers, but if they keep up with sewing or quilting and out grow the machine, by then they'll know what features they'll want on their next machine.