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How nice of you to research your gift. Your choice will depend largely upon how many people she usually cooks for.
I have the 8 quart Ultra (I usually feed 9-10 people) and don't regret it at all. This is the only version with the knob and I love how easy and intuitive it is.
On Amazon, if you click on an Instant Pot and scroll down, there are charts comparing them. Like here.
I regularly use the glass lid you have to purchase separately. There are other gadgets you can get for it, like stackable pots and extra sealing rings (because yours will eventually smell bad), among other things.
You can just type in "Instant Pot Accessories" on Amazon and see what is available.