The early straight stitchers looks very nice, often marked "rotary" or something like that. Some are of very fancy design and look very different, but I'm guessing all of these are very reliable and should compare well to any machine. They have a motor pulley directly on the hand wheel, and it's usually easy to replace it.
The later zigzaggers with additional cam sets look very nice to, I lean towards the 50s and 60s models. By the late 60s you have too look at each model carefully to see if it's worth the time and money spent to get it up and running again. Some of the later vintage machines are well worth it even if they have a few plastic gears, but the all metal models have an advantage. I'm not sure when Kenmore introduced plastic gears, neither when they started importing Japanese machines. The quality changed over the decades, not all for the worse luckily. I'm replying very generally, I have been looking at the early straight stitchers from the 40s, and early 50s. It will be intersting what the experts will say
