I would look at features that she might want.
A needle threader
Adequate light
Specialty stitches
Extended table
Needle down
Lock stitch
Also consider where she will be sewing. How light should it be?
The Brother CS6000i, Sparrow 15, or Janome models are great. Juki has some (HZL 29Z and above models in that range too). One place one might look is Overstock.com. They have a sale on the CS6000i this week.
By the way, I applaud you for realizing that this girl's talent should be encouraged. I made my own clothes by the time I was 10 because I had caring family that encouraged me. It has made a HUGE difference in my life! I wish Sydney the joy I have had.