zerowaste

On top of echoing what others have said-- you need a dumb sewing machine, and that socks are best darned by hand-- I'll give you some advice for finding a sewing machine workhorse. Long post ahead!

Do not get a new one. Get an old one from wherever people sell used stuff near you (For me that's gumtree/preloved). I'll be honest, I don't know when the quality drop off started decades-wise, but at some point they stopped building last-forever workhorses and planned obsolescence showed up. I think anything pre-80s is safe, and the good thing is if it's still around and working after 40-50 years, it will likely continue to do so another 40-50 with proper maintenance and possibly without.

Here's what you want to look for in your vintage sewing machine, apologies if you already know some of this:

• a straight stitch setting, which I don't know why I'm mentioning because literally every machine since dinosaurs does a straight stitch. But, this is what you'll use for non-stretch sewing. The length of the stitch can usually be adjusted.
• a zig zag stitch. This might come in the form of a dial you can use to adjust the "width" of the stitch, so you can have a narrow zig zag or a wide one. Honestly, you might find both the dial-adjust kind and a machine that just has the zig zag as a thing to select. I'd always go with an adjustable one. Zig zags are how you get stretch.
• If you're lucky, your machine will have a weird little hooked blade near where the needle goes. This is a threadcutter. It is second only to automatic threaders when it comes to "funny doohickeys they put on sewing machines". But you're way more likely to find a threadcutter.
• in general, a machine will be heavy (they had metal parts in the good old days), will have it's own motor and cable (you can find some old old singers with motor attachments, but I can't vouch for them), and it might be ugly on the outside, dented/scratched/etc, but it should function relatively well when you test it.

The last point being to say, test out your machine before you buy! It might not run perfectly the way the person has it set up, especially if this is just someone selling grandma's old machine, but there are some tests you can do. You'll want to bring your own thread, a new needle (these fit pretty universally AFAIK), and two pieces of fabric: stretchy and not-stretchy (woven).

Fit the new needle, wind a bobbin (or have one you brought), thread the machine yourself, put the bobbin in and try sewing a straight stitch on the woven fabric. You might end up with some weird stitches (looser on one side of the fabric for example) but this is probably that the tension isn't set exactly right for that fabric and not an indication of a bad machine. If you're getting big wads of the thread on the underside and/or the machine is jamming, it might be an indication of something dubious with the machine, but it's also likely something is threaded wrong or the bobbin is having a bad day. Honestly, I'd err on the side of "the machine is dirty or it's user error" if the machine is sewing, but poorly.

Next you'll want to try sewing on the stretchy fabric. See if you can set a zig zag stitch, and see how easily the machine feeds through the fabric under the needle. I'm out of my depth here on why, but some machines just feel better on stretch fabric than woven and others are the opposite. I'll note here that I think you'll be fine testing slightly stretchy fabric with the same needle you used for woven, but when sewing for stuff you want to last, you'll want to use a ballpoint or stretch needle. (Stretch needle comes out for any fabric with a lycra/spandex content over 5%. Ballpoint is anything knit (not woven essentially) but that isn't too spandex heavy.)

So assuming you find a good machine, I recommend getting it serviced as soon as you have it in your possession. It likely needs to be oiled and cleaned, and there are professionals who do great jobs at this, and they might even give you pointers on how best to keep your machine clean and happy.

Once machine is home from the sewing doctors, go forth and sew! Keep it clean of fuzz and loose threads and occasionally take it back to be serviced (I can't say how often because It Depends). Also, heavier material like denim = needle with a higher number in front of it on the pack. With the right needle, you'll be surprised what you can sew through.

Also, welcome to the hell of "I don't want to use polyester thread because plastic is a scourge but my machine hates cotton thread very much." I swear every machine hates cotton. Good luck.

Also also, because I am a creature of hyperfixations, you mentioned FOSS and sewing in the same sentence so I'm obligated to link you to freesewing.org. FOS sewing patterns that scale, theoretically, to any size!