1st) Yes the ipad 4th generation aka ipad with retina display, is 3G compatible,
Link 1,
Link 2
2nd) No you don't need to purchase it from the US to communicate with her brother.
3rd) If she wants to use Facetime to communicate with her brother then it is better to stick to wifi
4th) If device is only used for Facetime, then it is better to get iPad 2, iPad 3, mini iPad, or iTouch