Nanna, Joe has already answered most of your questions; as for this one,
"Would this machine sew several layers of jean fabric, for example?"
Imagine I told you I'm buying a used Chevy, and asked you "Will I be able to drive this up a hill?" Your answer woud likely be "Well, it depends on the condition it's in." If the machine is ship-shape, no problem. If it's rusted and the motor is burnt out, perhaps not. Best thing is to test it out yourself before you spend any money.