I have an Elna 7200 which is very similar to a Janome MC6500. I also have a Singer Quantum Futura CE-200. Both have the needle threader qizmo but it only works well with certain sizes of needles--smaller on the Singer--larger on the Elna so that feature isn't as important to me as the auto thread cutter--especially for paper piecing. All those thread tails will drive you crazy if you don't snip them right away!
The needle up/down is useful while sewing long strips together. It acts as a 3rd hand, holding your pieces while you line everything up and keep lining it up all the way down the seam especially if you hate pinning as much as I do.
Dropping the feed dogs is a must if you plan to free motion quilt. My Elna was purchased for frame quilting so that feature was a must.
The straight stitch plate is nice if you've been having trouble with things getting sucked down the hole but it's not a deal breaker for me. I only put mine on when I'm having trouble. My needle has to be adjusted over a smidge to use it or the thread rubs on it and breaks which is more annoying than occasionally trying to eat the corners on triangles. Leaders & enders help with that problem as well as a straight stitch plate.
My Elna also has the knee lifter which I rarely use. I realize it would be easier for some things but I just never got in the habit of using it. I learned to sew on a machine that had the knee speed control so now it's stuck in my head that that's what the bar is for and it just ends up p!ssing me off when I try to use it properly--LOL!
I wouldn't worry too much about the feet unless you buy some odd machine that needs special feet. Many take the generic snap on feet which can be purchased very reasonably. I can't be without my 1/4 inch foot but it's a snap on so if my original one disappears--it's easily replaceable. Same goes for the walking foot and free motion/darning foot although they aren't "snap-on". They are still fairly generic and work on many machines.
So, I guess auto thread cutter and dropping the feed dogs are my 2 must haves but it depends on whether I have a backup machine also that can do zig-zags or other fancy stitches. If you can only get one machine--spend a little more and get one that can multi-task.