There are two things I want to say. First off, I was 24 when my Mom and Dad had my youngest sister. Mom was 46 when she was born. She has been the apple of my parents' eyes and filled their nest when it would have been empty.
Secondly, on my birthday in 2001, we called our children, who were in Russia for the two months in between us meeting and hosting them in our home and finalizing our adoption. The caretaker who answered the phone that day said, through our translator, that she wanted to tell us something that probably no one else would. She told us that before Kristina had met us, she was uncooperative and wouldn't volunteer for anything. She didn't want to sit on anyone's lap or give hugs to anyone. She was 5yo then. BUT, after she met us, she returned to the orphanage as a changed little girl. She became affectionate and cooperative. The caretaker told us that this was because she knew that she was going to have parents who would love her and take care of her. She knew that even though so many miles were separating us, we were still calling her and letting her know that we loved her.
Why do I tell you this story? Because in your OP, you have already said the same thing. Isabella was underweight and malnourished. She was being put in harms way time after time. Then you came into her life and filled it with love and HOPE. As long as she has you, she has everything!
So, follow your heart and do what is right for that little girl. You don't have to have a lot of money to make a child feel loved and protected. I grew up pretty poor and VERY happy. My DH grew up with money and had a miserable childhood. Just because someone has money doesn't make them a better parent. The same thing applies to age. Just because you'll be an "older" parent, doesn't mean that you can't do a wonderful job parenting.
God Bless you, your DH, and Isabella! I'll be praying for your quick and easy adoption.