If you are only looking for a good straight stitch, I would skip all of those machines and purchase a vintage Singer in good working condition. Check your local Craigslist, E-Bay, and GoodWill stores. You should be able to pick up a machine for $50 or under that needs some cleaning, or a machine that is cleaned and ready to go for under $200 ... with a cabinet.
Models 15 & 15-91 are good strong machines that can sew a beautiful straight stitch. The 15-91 is gear driven (no belts) and providing you keep it in good working order (oiled and cleaned), it will go forever. It's a fairly large and heavy machine. Another fantastic full size Singer is the model 201. I wish I had a 201 in my collection.
The model 99 is a smaller (3/4 size) machine and is also a nice machine.
The 401 and 221 (featherweight) are smaller and lighter weight machines, but both sew a great straight stitch. The featherweights are harder to come by at reasonable prices - expect to pay upwards of $200. The bonus for both of these machines is the portability - easier to travel with.