You are all right about the FW, 201, 15-91, 301 Singers for straight stitches, but the cheapest one I have run across is a Spartan (heavy heavy heavy), but so quiet and totally dependable for the 1/4 inch--I think mine has the marked plate, but I am not real sure about that--I don't have much trouble with keeping my seam at 1/4 inch after I work at it a minute or two, I don't need the indicators anymore. But I can't depend on keeping my seam true at 1/4 inch on my newer machines with the zig zag foot, even when I put the straight stitch foot on them. Like they are saying above, it depends on the individual doing the sewing (and maybe her speed, I am not a speed demon on a sewing machine).

a straight stitch machine will always sew straighter than a zig zag machine. I piece most of my quilts with a FW or treadle machine.

So true. Just tonight I noticed that my left arm twitched just a teensy bit and my 1/4 inch seam became about 1/16th of an inch.

Have you purchased a 1/4 inch foot with a guide on the side? That will help with straight seams. They're pretty inexpensive, too.

I used a 'cheapo' Brother Machine for 5 years and just upgraded to one of their better sewing machines. I do see a difference in my stitching consistency. But I think that had I purchased the foot described above and used it, the consistency would have been much better.

what is the difference between the Singer 15 and 15-91?

Another nice, straight stitch, vintage machine is a Singer 404. It is part of the Singer Slant 400 series. It is a full size, gear driven (no belts), horizontal top loading bobbin, full rotary hook, Aluminum bodied, take no prisoners machine. It is light weight at 19 pounds, and fast because it does one thing and does it very well.
You can usually find them for a lot less than 100.00 because people don't really know about them. Sweet machine.