I have both a Singer 15-91 and a 201-2, as well as a Red Eye (66) and a adorable 99 in a bentwood case. You have to think, what are your goals, do you just want a couple? Or you planning on collecting? If you want 1-2 main machines, if they move freely, have at least some kind of accessories and possibly the manual, it's well worth $150-$200 for a solid vintage machine, that you can sew with for another 20+ years. Often at garage sales you can score a vintage machine for much less. Plastic machines will be a headache and not worth the money in the long run. These old machines were not meant to be disposable. They were meant to be repaired and serviced by the women and men who relied on them back in the day.
When I got back into sewing my mom offered me her Necchi Supernova and it's a fine machine. But it couldn't do the denim straps I needed for my fun upcycled denim jean bags. I joined a facebook group to learn all I could about these fabulous machines and how they work. Not only to figure out which one I wanted to look for, but also so I can encourage others to pick up the torch of admiration of sewing and the tools it takes to make beautiful things.
I mentioned my potted motor machines (the Singer 201-2 & my 15-91) as they are often the first ones a person will get after researching the models. The later slant needle models, the two toned really cool space age looking ones, those are cool and some use cams that offer neat decorative stitches. So think about what you want from your journey in to the world of vintage machines. You can come to that huge open facebook group and get help from members to get through most any problem you can have with these old machines.
I recommend learning a little about them before making the plunge into buying one. Know the parts of a machine, like some have drop in bobbins, like class 66 metal bobbins, and some use a bobbin case that clicks in. I just bought a Kenmore 158.1941 and it was missing not just the bobbin case, but also the hook, but I got a great price on it in a cabinet, and took a chance I'd be able to buy a replacement hook. Which I found, along with the bobbin case and it now runs great! My point is I've seen machines in shops that were missing parts and you can't always replace them. For instance the tension assembly, if that is missing, you should check online before buying the machine, that you can get a replacement. But I was able to install my machine's new hook and had it sewing within ten minutes, most of that was due to learning how to thread it and it's bobbin case.
That's the beauty of them, they were made to be simple and user serviced. I personally had to get both of these two models, my 15-91 was frozen solid, but with the members' collective knowledge and encouragement I got it moving again! It will sew again! I haven't made a bag on it yet, but I've made some nice hats and a couple of bags using Beastie, my 201-2. Biggest thing is not to shy away from asking for help or questions. That's the only way to learn and gain confidence in using and servicing your own machines. If the one you are looking at is missing the pedal, you can probably replace that or the bobbin cases, but be aware, the Singer 301's and Featherweight's bobbin cases are harder to come by, and if you want one, make sure those have their original cases. I've heard there are reproduction cases out there, but that quality and performance of them can be hit or miss.
Now if you're seriously talking bags are all you're interested in making, you might want to like get an industrial, they won't break down like a domestic machine will after a steady 'diet' of heavy fabrics or leather use. And make no mistake these singers I mentioned above are domestic machines. Here's a great post on a awesome blog about the differences between a domestic machine and an industrial. Many unscrupulous ebay sellers or craig lister use the term industrial when listing a domestic machine and it's so wrong.
I hope this was helpful and if you have any questions I do hope you'll ask them!