resipsa re your comment....
... the PILs think we'd regret it and should be grateful for/stick at DD. I'm so indecisive since hit with secondary infertility that anyone's negative reaction makes me think we should not proceed. How do you all cope with others' negativity or indifference?
I really would not allow anyone else's negativity or indifference to affect my decisions for a minute (unless it was your partner's feelings). No one else is living your life but the two of you.
It really has to be your decision and not one your in-laws can contribute to. Of course they are concerned for your DD, their granddaughter, and it is only fair for them to express that. But really I feel 'grateful for/stick at' is not something you should need to give any space to (in my humble opinion)! The question is not are you grateful for your dd? Of course you are! The question is do you want to be parents again, have you enough love and time and energy (and money) etc etc. If you choose to go forward it does not in any way reflect on your love and gratitude for your dd.
Maybe you are allowing their fears to affect you as they mirror your own. I think you have said as much already. My advice would be deal with your own fears and concerns, realistically (as you are by engaging with us) and be ready when you know what you will do to explain calmly to in-laws. Ultimately, it is not them you must answer to, and to some degree not to your dd either, except that of course you hope this new sibling will add to her life and not detract from it. No one can guarantee that but that is the same with a birth child and you can minimise the risks by the kind of 'risks' you are willing accept in a new child into your family.
You asked how old birth/existing children were when new child came. Our dd is 9. She is struggling a bit at the moment but it is early days.
Here is my advice if you do go for it....
Go for the right type child for you (try not to have too many preconceived ideas about who might be right, just read the profiles and see if you and your Dh are drawn to the same child. try to open minded! It is hard! We has a strong preference for a girl but went for a boy, my Asia connections meant I would have loved a mixed heritage child but we are adopting a white child, all the things I felt were important (as young as possible - we ended up with one of the older children we considered) were all not as important as the right one.
Having said that I would...
Go for a big age gap if you can, as big as you can within reason, I think it makes life easier, unless both children are very young anyway.
Get lots of info before making your decision, so you feel prepared.
Do not feel pushed or rushed into a decision by anyone. once social workers leave the scene you will be left with your choice and so it must be a genuine choice and not because you feel sorry for a child or because social workers put pressure on you (IMHO).
If you are adopting as a couple you need to agree, if you can't both agree on the child I can't see how that child would be right for you.
I personally do not feel there is one right child for you, a bit like marriage I think there are many children any of us could adopt and we end up adopting a certain one or two and it is great. People have already said to me stuff like 'he was the one for you' and I agree, he was, but there may have been others equally 'suitable', it is not a competition to find Exkhaliber's sword! Iyswim!
Good luck.