To answer your last question.
No I would not buy a vintage machine for the purpose of doing free motion quilting. You certainly can do it however.
Tredle, to me are useless for FMQ as you can't get a consistent speed. the older electric ones work if the electrial system can be controlled with the foot pedle.
Buy a good used modern machine for this purpose. You will be able to get both walking foot and fit with darning foot for FMQ.
I don't believe you will be happy trying to FMQ on any of these.