In terms of the debt you will most likely not like my thoughts, but here goes! 
I think you and (most of all) your dh need to address this issue of left over student deb now held in credit cards. To me, this is a very big sum, maybe not to you, but I think to the average person it does sound a lot.
It sounds like he works part time, you have a lot of expensive holidays per year yet he still owes quiet a lot (to me) of money from the past. This is tied up in credit card debt, so not the same thing as a student loan, I think.
You might say why does it matter, who cares. But I think it is a sign that there is something from the past, he has not addressed and is in no hurry to do so. To me it is not a good sign.
And if he were to get into more debt, for whatever reason, this could potentially cause issues for him, you and any children you adopt.
Because the children coming through adoption services have had a really shit start in life, they deserve to have nothing else unnecessarily rock the boat.
At any time we could get ill, get into debt, lose jobs or have businesses fail. So that could happen to anyone. But to start a new relationship, of adoption, already quite a lot in debt with a partner who could pay that debt off quicker but chooses not to, seems to me an unnecessarily risky place to be.
I am not saying you should pay the debt. I am saying he should address this, especially because you have been told it could affect your chances.
It is a competitive area.
If your husband is not addressing (or making a bigger effort to address) something that could affect your chances, a social worker might conclude that adopting is not as high a priority to him (or you as a couple) as it might be.
Is it possible he is less keen to adopt? It would not be the first time one partner was ready before the other.
Good luck. 