One thing to mention is plastic vs. metal machines. Some of the new, inexpensive computerized machines do not have a metal interior frame for support. If you drop them, or if they are exposed to excessive heat and warp, they won't work and some cannot be repaired. Even normal use means that they can become misaligned over time and repair is usually not worth it.
FYI, Sears is no longer branding their sewing machines [at least not in the US], so Kenmore machines are now only available used.
You have a valid point regarding metal frames, although a cheap sewing machine is probably going to have other issues that limit its longevity, too. Thanks for mentioning that, though. And thanks for the info about Kenmore!