This assumes almost no knowledge whatever, and no mechanical ability:
For those scouting old sewing machines / thread injectors, those great all metal engines probably built to run 10,000 hours or 100,000 hours of hem and seam, which ever comes first:
In looking at some previously-owned machines, and in learning to use one/several, I've found bobbin holders and hooks that have scratches in them, and the scratches have not always been repaired by sanding and polishing.
You're going to have to know how to remove a bobbin holder and the hook to clean that area, no matter what machine you operate. Learn to do that on any machine you can, and learn how smooth and polished those parts are: the bobbin holder, and the hook.
Some of those scratches have been put there by inexperienced and untrained operators, like me, who thought they could sew through obvious missed stitches and collections of thread in the bobbin area. Wrong, and with lousy consequences: The needle gets deflected, goes where it shouldn't and scratches the bobbin or hook, especially the hook right after the point.
Why that matters so much: This isn't cosmetic. Smooth passage of the thread around the hook and over the bobbin is absolutely essential for 100% complete stitch formation. Find any imperfections which MIGHT snag thread, and you may have found THE REASON why someone gave up on the machine. Not just missed stitches, but repeated bunching of thread in the bobbin / hook area. You may have on your hand a real find, all other things being equal.
In other words, with no mechanical know-how whatever, you may be able, with wet-or-dry sandpaper, 600-1000 grit and 5 minutes of patient rubbing, to remove the only reason someone gave up on the machine as much as 30-40 years ago. The difference between a machine that does not skip stitches and one that does can be like the difference between the typical magazine photo of a model, and the real thing. Cheaper than Photoshop or Gimp, and without 1% the skill to use those tools, all that may be required is some rubbing. (So there's no confusion here <smile> you don't want the REAL model. You're after the one that has had all the imperfections and blemishes removed.)
If you can get the current owner to operate the machine and produce some lines of stitches, hopefully with some skipped ones, you can look for lint in the bobbin area, and especially those scratches, all without saying exactly what you are looking for; and you may be able to strike a better deal than you hoped for, the machine needing nothing but that repair you can do yourself.....and probably a lot of patient re-oiling to dissolve old varnishes and re-lubricate those bearing surfaces.
I'll add, for further negotiation and economical shopping: Replacement hooks and bobbin cases of known top quality are as much as $15-$20 each, or more. I don't know enough to say whether gouges that have been sanded smooth can then be filled, sanded (and lacquered) for repairs that are good enough to last a few dozen (or hundred) yards of stitches.