OK, here's the deal from an adopted mom who also gave birth to a 'baby of the belly.' I also have adoptive mom friends. If God wants you to have more children, he'll put options in your path but IT IS ALWAYS UP TO YOU to use FREE WILL. He gave us that, too. So the God issue is a red herring.
It doesn't matter what the race. It doesn't matter what the condition. It doesn't matter what the medical history. It all just doesn't matter. Your kid is your kid. Friends of ours have two girls adopted from the same country. Other friends have a Guatemalan-born son and Korean-born daughter and they are white Vermonters. Does not matter. When God gives you a baby of the belly, you don't know what you are getting really. Notice those TV shows that follow folks families back to previous generations and found out all kinds of stuff they never knew. Doesn't matter.
The baby from my belly has had more medical issues BY FAR, including a thing (that just happened in the womb and had to be dealt with when he was 9 mos old with surgery), while my adopted baby has never had a blip or a cold. Needing parenting is not the same as a potential lemon in the used car market.
So, if you want to make God the fall guy, don't try to select the details -go for the opportunity that feels right in your gut.
On the other hand, if somebody (your hubby) doesn't want to adopt and for him it's about second choice instead of "a baby of our own" it just may be that an adopted child will be treated by him as a second class citizen. Who deserves that??? On the other hand, once you have the baby it is yours. Period. The rest really, really, really doesn't matter. (Y'know if God wants you to have another child, perhaps he wants you to make the choice about finding the child who needs your love the most: maybe a Haitian orphan? In terms of which kid, make it about the kid rather than you.
You did the right thing looking here for information. That fear you describe is probably the biggest thing you have to worry about. So: read up on adoption -you AND your husband. Your questions show that neither of you know much about things in the adoption world -that your fear is overwhelming. Other people who have not adopted, whatever their good intention, are not the people who know, so avoid asking them and if you get free advice, say thanks and remember if they haven't adopted, they're guessing. Obviously. In terms of whether or not to adopt, make it about the facts, not the worries. Birth babies bring problems we're unprepared for too. That's my advice. Be well. Good luck.