I guess my question is what will happen to the teenage mother when after she gives birth? As soon has she delivers will you kick her to curb or will you continue to help her out. If your just going to use her to get a baby, that's pretty terrible. What if she changes her mind after the birth and wants to keep the baby, will you accept that. You need to look at the big picture, the girl you take in may decide to keep the baby.
I've read your earlier posts, you have issues that need to be resolved dealing with your furtility before you consider adoptions. I'm positive you will make a wonderful mother one day and offer a child a wonderful home to grow up in. My concern is that you seem to be looking to fill a void caused by your inability to give birth to a child. You need to resolve that issue before taking in a child that may not fill the void if it's still there and will only suffer because you resent the fact that the void in your life is still exists.