Hi not adopted but I spent a lot of time in care homes. Sleep may well be an issue when everything in this child life changes, I used to lay awake heartbroken, feeling alone, missing my mum, dad siblings, wondering when/if I would see them again, wondering if I'm in a safe home, wondering if I'll be in this home for a long time or not.
If you have a kid there will be sleep issues, my youngest struggles with worms as he sucks his thumb, that's a no sleep until 3/4am issue. He is a good sleeper otherwise.
My honest advice would be get over the sleep issue, all parents have a period of learning that you can survive on fuck all sleep for a surprising amount of time.
A 'difficult' sleeper is likely to be a very lonely and unhappy child at night, you're alone at night, your head can tell you things that are awful. It's also possible that you will be the first person to try and have a 'bedtime' for this child.
You need to be very aware that children waiting for adoption will change over time. They will be a bit fucked up, a child who appears to be happy will have issues that they may not be aware of until they've been in a stable home for a while.
You're not getting a sheepdog who has been trained you are getting a child who has no idea whats going on. Your habits are new, you are new, you may not be there in a month, you may be yet another adult who doesn't like that child. It is so hard as a kid to believe that you are not in care because you are bad or unloveable.
There is a reason it takes so long.