Look for about any machine that is ALL METAL - inside and out. You want to make sure it makes a stitch. I think there are pointers on here other places what to look for. Are you looking to collect or just a good very basic machine? The Singer Featherweight is very collectable and so is the Singer 301. If you want something inexpensive look at Singer 15 clones - they are made in Japan and work very well thank you very much. I think if you go up to the top of the page and search you might find pictures and a ton of info. There is blank box that says advanced search. Singer 66 is a very simple and good machine. White machines. Elna, Viking, Bernina, the upper end Kenmore, some of the Necchi machines are fantastic, Pfaff , New Home, Free, Domestic, Davis, and the list goes on. The old machines were made pretty simple and will last a very long time if they aren't all rusted out - that said, there are members who take those old rusted up machines and fix them up to run. There are many old companies that made fantastic treadles. Peek on the vintage photos. You'll see tons of machines. My all time favorite is a Singer 403 which is similar to a Singer 401, 500 or 503. You can search the same above and find all kinds of posts. Yes the machines went into decline as soon as they went plastic. I would say our vintage shop talks about any of the old metal machines. As far as time frames - there were still some metal machines made in the 1970s.
http://www.quiltingboard.com/vintage...s-t130994.html All kinds of old machines.
http://www.quiltingboard.com/picture...p-t164361.html - all around good machines and cheap too.
Here is another link
http://www.quiltingboard.com/blogs/v...nks-b7514.html
Actually something you never heard of might be a really good machine - do some home work on here - there are tons of things to read.