Don't limit yourself to just a Singer Featherweight. There are lots of other fantastic sewing machines in the wild, too. We have a lot of info on Japanese machines here:
http://www.quiltingboard.com/picture...p-t164361.html and for Japanese machines that zig zag:
http://www.quiltingboard.com/vintage...s-t207877.html and Japanese machines that do more than zz:
http://www.quiltingboard.com/vintage...s-t207880.html Then there are photos of old machines:
http://www.quiltingboard.com/vintage...s-t130994.html not all of them are treadles. Here is a link about smaller sewing machines:
http://www.quiltingboard.com/vintage...e-t185449.html Many of the older machines are a joy to use - they are so simple to use and maintain. I have sold a few machines to people who figure for the cost of one service call on the electronic they can use vintage for straight sewing save the electronics for other stuff. Believe it or not you can still find parts for a lot of the machines.
If you are just wanting information how to evaluate a sewing machine, I think first I would google search 'how to evaluate a vintage sewing machine' and see what you get.
You can buy cheap machines a lot of the time - I buy them with out checking them over much, take them home and go over them. Some times I've been fooled into thinking one was fine only to have it so stuck it won't turn or to rebuild a tension. If you don't want to have to do repairs, you might have to pay more. Download a manual and read it. Then, I would sit down with a machine I was interested in buying and spend some time. Sew like you normally would. Take your own fabric, thread, needles, scissors along and sew. Spend some time to be sure it stitches nicely. Check for rust, check for dried oil - They look the same some times. Check the wiring. If it doesn't do what you want it to do then don't buy it.