I understand about being overwhelmed by choice. I used to be into every craft I could think of, and I had the supplies to show for it. I pared myself down to sewing clothes, making jewelry (beading, metalwork, enamel, etc), and making collaged greeting cards when I need an easy project. I'm much happier for it. I also get overwhelmed when I have too much stuff, but I like having the needed supplies to make my hobbies fun, so it's a fine line!
It sounds like you are thinking this way already, but if you don't use the coverstitch and don't really intend to, you probably don't need it and you might only need one serger, depending on functionality.
What do you think about putting away whatever sewing machine(s) you are having trouble parting with and see if you miss them after 6 months or a year has passed? If you find you've needed the machine(s) while they are put away, that tells you something. If you haven't needed them, maybe you can sell or give it away without worrying.
At any rate, that's probably what I would do if I was having trouble deciding; I do it with items that I'm not quite ready to part with sometimes.