I started using vintage machines in treadles this summer for sewing quilts when I purchased my first treadle Singer 66-1. All my machines have been found on CL or free. I have around 10 machines now, but there are only three that I sew with, and yes, they are in treadles. I took their external motors off of them. My all time favorite machine is my Singer 201. It is the smoothest and quietest machine with the prettiest stitches I have ever seen in a machine. It is great for piecing and straight stitching quilting, but I don't use it for free-motion quilting. My 15-90(in treadle) is the machine I use for free-motion quilting. It too is a great machine for piecing and straight stitching, but not as nice as my 201 for this type of work. Why the different machine for free-motion use? It has to do with the bobbin area and the way the pick-up of thread for this type of quilting. The 201s and the 66s have a horizontal bobbin area while the 15s have a vertical bobbin. It seems that the 15s are better for free-motion quilting because of their vertical bobbins. I would hunt for Singers since parts are easy to find for replacement, and I wouldn't pay more than $50 for treadle and machine that you are going to sew with. I picked up a free treadle with a crappy cabinet(with machine Singer 9W1) and later a free cabinet. They can be found, just keep watching CL. Yard sales are great too for cheap but awesome finds. Also, reading the Vintage Sewing Shop that Billy started here on this site is very informative and educational. Plus, fun to see other folks machine finds and projects.
My 201 has a walking foot on it that I use all the time. It is designed to fit the feed dogs of the vintage Singers.
Singer 201 Tension in front, longer thread holder
[ATTACH=CONFIG]143240[/ATTACH]
Singer 15-90, tension on side
[ATTACH=CONFIG]143241[/ATTACH]
Singer 66-1 with brown Lotus decals
[ATTACH=CONFIG]143242[/ATTACH]