Just wanted to add some thoughts regarding testing a sit-down.
If you can, take several different thickness quilt sandwiches to see how easy and repeatably you can set the foot height.
Try different weight threads and different needle sizes to see how "fussy" about thread it is.
If it comes with a bobbin winder, wind the bobbins for each thread using it, then stitch with them for a while (or stitch, then remove a bunch of thread from the bobbin and go back to stitching) to see how evenly they were wound.
Run the stitching at various speeds. Can you "outrun" the machine (especially the stitch regulator) and end up with bad stitching? IIRC, the Bernina uses domestic needles, not longarm needles. These are thinner and may deflect more if you're moving the quilt fast. Does this happen when you are stitching at your maximum comfortable speed?
Speaking of which, since the Q20 has a built-in regulator, try stitching with and without. Make changes in direction, tight curves, stitch at random speeds. How well does the stitch regulator perform? How well can you quilt without it turned on?
Take a bed-size quilt sandwich (or finished quilt). Quilt (or pretend to quilt) across the whole thing, pushing the bulk of the quilt around on the table, up into the throat, back and forth, left and right. Does it move easily or drag or catch where it would cause problems/frustration?
Are they including a chair/stool or will you need to get your own? How is the visibility when sitting in the intended chair/stool? Can you achieve/maintain a comfortable position and see your work clearly? Do you have to hunch to see what you're doing? Is it fatiguing at all? Ergonomics are very important when you're going to be doing a task for hours and hours, even with stretching breaks.
How loud is the machine? As you are stitching, particularly with the regulator on so the speed is always changing, does the change in sound bother you at all?
I'm sure others will chime in.
Last edited by mkc; 12-12-2019 at 05:27 AM.