Please do your research on the last few years' high end machines and watch carefully how you use your sewing machine. Notice what features you really value and use a lot. Think about the times you thought how nice it would be if your machine only did "so and so." Asking other serious sewers questions as you did here was really savvy.
I bought an Elna Super in 1972 (top of the line then) and used it until it absolutely wore out (2010) and the company no longer supported it. My daughter gave me hers and it's ready to go in my sewing room. That machine has features I haven't seen in any machine - ever. During the year a wonderful dealer and I tried to get the Elna fixed, I bought a Bernina 1630 - high end some 18 or so years ago (I think. Anyway, I am the 3rd owner.) It's a wonderful machine and allows me to do some branching out. I like it a great deal and do use it about half my sewing time.
I've attended a get together at the Bernina dealer, showing off the newest, greatest Bernina. Interestingly, the features on that most current, high end Bernina machine had the same features the 1630 had: the screen on the machine looked different; the newer machine had more stitches; there were other differences but not, for me, substantial. Compare and contrast a few years ago's machines with the current ones. What you really want could be substantially cheaper on a lovingly used, older machine.
Before you get to the buying point, double check what's for sale used. Someone else's "used" and my "used" are not the same thing and there are wonderful buys out there - my Bernina for example. Please make sure before you invest in a used machine that it is still being supported by the company that made it. I just about spit acid when Elna America told me that there wasn't even a repair manual in the US for my Super, forget parts.
Enjoy the hunt and please let us know how it turns out.
Pat