Justtoobad I wish you all the best for this and that you will get the answers you need.
I also want to sound a bell of caution, taking a new child into your home, either adopting them or fostering them (but especially adopting them) could be very difficult for your birth child.
If you start down this route and then change your mind do not be afraid to say so. Do not assume you will not be considered because you are not related, you may well be considered.
The 'normal' way round of ensuring your birth child (if there is one), or other adopted child/ren already placed, are older than newly arriving children in the family is for valid reasons. It means that the probably more troubled (behaviour-wise etc etc) child is the younger one.
I know you know that children coming through the care system will have had difficult experiences, and may have behavioural issues. By being the younger child it means that hopefully those problems can be contained and helped more easily.
Also, the already placed/or birth child is more stable and settled and is older, so can in some ways can forgo the immediate attention young children need and have the benefits of being the older child. In that they get that extra time when little one has gone to bed etc to build that bond because little one demands time during the day.
By the younger one being the more stable and established one you may find they are squeezed out because they do not always need the immediate attention and they do not get the benefits of being the older one either.
By adopting a child much older than a birth or existing child there may be tensions (I am putting it mildly) between the needs of an older, troubled child and a birth child, who may feel a bit pushed out.
I am sorry if this is all very obvious but you did not mention this in your post.
I am an adopter with a birth mum, my kids are growing to love each other but there has been a lot of tension and jealousy. My birth dd is 10 and my adopted son, 4. To some extend I am a police woman with them, ensuring that they do not fight and argue. It is very stressful.
Just wanted to share this with you.
All the best.