Well, I have experience growing up in an adoptive family with children with special needs.
My oldest brother has FAE, RAD, ADHD, and severe emotional issues and mental illness. He also has very complex health issues. When he was adopted, he spoke only French, and my parents spoke only English.
My 2nd brother has a developmental disability related to brain injury from abuse. He also has RAD and emotional issues. When he was adopted, he was being taught sign language because no one thought he would ever speak.
My sister has ADHD, RAD, learning disabilities (a bit beyond the typical person with ADHD), and emotional issues.
I have emotional issues from being adopted...which many (if not most) adoptees will have....they were nothing serious while growing up, though. I also was designated "gifted", so had special needs when it came to education.
My mom raised us on her own. My "dud" was around for the first 10 years, but didn't really do much. My mom has been to hell and back, especially with my oldest brother. He recently returned to our lives after an 11 year absence.
It was a LOT of work raising 4 children with special needs. My mom was a SAHM, and did a lot of advocacy for all of us, as well as speech therapy, behaviour therapy, home-teaching, etc. for all of us. There was never a dull moment.
My mom found it highly stressful, and didn't always handle the stress well, but in the end, I think she did a darn good job!! My oldest brother graduated from high school at the age of 23, and is now a University student. His interests lie in politics and technology. He works as a freelance tech support person. We think he may be an alcoholic, but he is working through a lot of his issues with his natural mother, with whom he has been reunited for the past 3 or 4 years.
My other brother lives on his own, holds down a steady, full-time job, and takes care of his 3 cats! He has a daughter, who was adopted at birth.
My sister just recently ended her mat leave, and now has a full-time job. She lives very far away, but calls us at least once a week. She has a daughter, and is a single mom.
I'm a single mom, too, and am in University taking nursing. I previously earned a diploma, and was working full-time in my field up until I had my son.
So, as you can see, it's not all been roses and butterflies. BUT, when you look at what the social workers expected of my siblings, my mom worked miracles.
The work is INCREDIBLE....the reward is unmeasurable.....or so my mom would say!

HTH!
ETA: I realize that it might seem like a child with Down's Syndrome would not be as complex, but unless you adopt them from birth, they are likely to have additional issues....heck, even if you DO adopt them from birth!