That would depend on who she would be stitching for AND her tastes. Thankfully there are many different ones. My favs you have mentioned already. Tell us what she likes. Maybe have her look through projects and see where her interest lies.
You picked my favorite sites already. Maybe Mygardened/Yorikos could be added. The membership sites are the most cost effective.
The MC 15000 has an excellent selection of built-in designs. Better than any other machine I've seen. There are a variety of different styles so that she can get to know how they stitch (and how long they take!) It would be good for her to explore these and make friends with the machine before she buys designs because they look nice. I know I have many designs which I like the look of and only discovered the stitch count when I was thinking of using them. Most are sitting in folders unused!
I agree, top end machines tend to have top designs (and a lot of them) built in and they are the cheapest way to get acquainted with your machine.