Want to adopt 17 year old Bulgarian. Anyone have answers?

I know a girl of age 17 (just turned 17) She came to America with a visitors visa which has expired. Her father is here to look after his son who is a minor, but is an American citizen. This girl is very bright, sincere, and hard working. Attends high school, makes good grades, and is an "overachiever" I care very much about this girl. I love her like a daughter, and my husband and I are of the same mind on this. I wish very much to adopt her, but I have some concerns: [LIST] Is she too old to be adopted? Would she have to return back to Bulgaria in order for this to take place? Would the fees to adopt in this manner be as costly as going through an adoption agency? [/LIST] Is this the best place to post this question or should I re post else were? Any help is greatly appreciated. Thanks so much God Bless