Singer is one of the most popular brands so will be easiest to find parts for. Husqvarna and Brother are slightly less popular but still pretty popular. I’ve never heard of any other the other brands you listed. Some parts, like needles, are standardized. Others aren’t.
You can’t sew with two needles at once, but there is such thing as a double needle that’s used for special cases. More than likely you won’t need one. The second spool holder is most likely for 1) use with a double needle, 2) quickly swapping out thread colors or 3) actually a bobbin/spool winder and not actually for holding a spool you’re using.
As for stitch pattern symbols, each symbol is supposed to look like the stitch it creates. I would recommend trying to find a video showing all the stitches for whatever machine you pick. If you can’t find such a video, you can experiment with some scrap fabric.