If you don't have deep pockets and want a good retro machine buy an old all metal White - the older all metal Japanese machines are pretty cool as well. They will have names you have never heard of. There are some awesome old Kenmore and Necchi machines. On the other hand I've seen some old Kenmore machines ain't worth a plugged nickel. One we had back in the 60s was the very bottom of the line and just didn't get used much - nobody loved it - it only did straight stitch. Montgomery Ward made some nice high end type machines too.Look them over and see if you see ANY plastic gears or moving parts. They get brittle or what ever after time. Bottom line - look for metal parts.
Another thing to consider. Sit down at the machine and make sure you can 'see' the needle without craning your neck. Is the tension in the line of vision? Listen to the machine. Does the motor sound good? Does the machine clank and sound funny? Some times it will sound funny because there is lint gunking up the bobbin area. Or is it someplace else??? Has it been oiled? Do you see dried up thick gummy oil? Does the tension work? I recently got one cheap because it had a tension wire spinning around. Does the zig zag work? How much wear and tear has it had? Check to see if the thread comes up from the bobbin. My sister got a 301 very cheap because they had the bobbin cover messed up. You can learn a lot by reading up on the trouble shooting stuff on this board.