I don't have experience with the birth order thing, but we did (finally! it took 3 years!) adopt a girl who is now 9, who has lived with us since she was 6. Her brother is also with us, and he was 2.5 when he moved in with us (5 now). For me personally, the hardest thing lately has been coping with a whole lot of grief. Grief over missing out on their early years (and having a 2 year old dd by birth highlights that at times). Grief over the trauma and neglect they experienced. Their grief over their past, their birth family, the life they used to know. It's been really hard work. Lots of therapy :) It's not insurmountable by any means, but has caught me by surprise.
The other thing that I didn't really know (but might be obvious to those who went the traditional adoption route. We didn't, ours was a surprise kinship placement) was the degree to which the children's trauma still affects them today. It truly does pervade every part of their being, every part of our day. We're still learning to cope with some of it and with our younger son, are only now really seeing the ways in which he's affected. It can be very difficult to know why a child is behaving a certain way: developmental stage or trauma? I think that the behaviors can be similar, though what they're truly looking for is very different.
I'd recommend finding lots of resources in advance. Not to scare you, but to help you be prepared to get your child and yourself the help that you may need. Knowing what to expect, from the minor to the possibility of major stuff, and knowing where to turn for help, seems like half the battle. We've certainly learned a lot over the past 3 years, and we've had beautiful moments in the process. I wouldn't trade it for anything!