You should NOT hesitate to cast this actress as Maria if she is the best person that auditioned.
To do otherwise is racist, plain and simple.
The Sound of Music does not explicitly mention race as a factor in any decision that Maria makes or that anyone makes regarding Maria, therefore it is not an issue.
Now that you have posted this, however, you leave yourself open to a possible lawsuit should you not cast this woman as Maria. Should you decide not to cast this woman as Maria, she certainly could have a case since you've just admitted that her race may indeed be an issue for you.
But you shouldn't have anything to worry about if you do the right and honorable thing, and cast the part with the best actress, if another caucasian actress appears magically and does a better job, then by all means go with that that actress, of course.
"Fenchurch is correct, as usual."
-Keen on Kean
"Fenchurch is correct, as usual."
- muscle23ftl