@aetios just can be diffucult to find the right bobbin when you don't have a correct one to compare to.
btw the serger will likely use a few threads at the same time. Big cone shaped ones. You can use them on normal machines as well. Might need some holder though. (easy 3D-Print tbh)
Also get yourself some new needles with a used machine and replace it after buying one.
Same might be useful for the overlock as well (though I dunno how easy its to change on that one)