I have the generic version of the Juki. I have a Chandler 8700. I have to say I really really like it. I don't sew that much at one time (I wish I had hours on end to devote to quilting!) but Chandler does what she's supposed to do! She sits in a nice, big table, and has an 11-inch harp. I'd recommend a Servo motor. The problem I ran into, and after talking to others, is that your local Juki dealer doesn't keep these in stock and they don't seem to want to sell industrials. Mine tried to talk me out of it and steer me to all the "more expensive" ones. I don't know what I'll do if I need repairs. I sure won't be taking her to the local guy!
They use the same high-shank feet that my "little" Pfaff uses and your grandmother can buy bobbins and needles online without much hassle. You can also adjust the pressure foot. I haven't tried to drop the feed dogs, I just cover them with tape.
The only complaint I have is that it was a pain in the hiney to put it all together (I ordered it online and it showed up in 3 boxes). There were no directions and we had to wing it. But I got a brand new machine for $425 delivered free to my house. I don't see how you can beat that!
I really wanted a Sweet Sixteen, but talked myself out of it (and various other "quilting" machines) because they don't sew a straight line. My "dream machine" is a Janome Horizon, but the only reason is because it has 11 inch harp, sews a straight stitch and a zig zag. That's all I really use! (yeah, I'd use more stitches if I had them....but....) so I do have to zig zag on Pfaffy so I really have my dream machine... I just saved about $3000 by getting Chandler and keeping my 30-year old Pfaffy when I need her!
I don't know if you've considered one of the 9-inch harp machines, like the Juki 98, Juki 2000, Brother 1500, Janome 1600, etc. They would also fit your grandmother's requirements, but a little smaller. They don't stitch as fast (1500 spm) but they also don't weigh 80 pounds and don't sit in oil pan or have a separate motor.
Hope that helps! If you're in my area, come by and I'll let you try mine out!