B.P.
Raising an adopted child will be no different than raising your own biological child if you adopt a baby.
I was adopted 1 day shy of being a month old and never remember not knowing I was adopted. My mother always said that some women carry their children in their bellies for 9 months but she carried me in her heart for 10 years (how long they tried to adopt). I have always know "who I was." Yes being adopted is something I'm proud of because it's unique, but it's never been anything that has hindered me. When I was in Middle School and High School I had a burning desire to find my bio parents but as an adult I'm actually kind of scared to do it.
You really have to tell the child from before you think he/she will understand. I think it's horrible when people hide it from their kids. I had a boss that did that and when they finally told him when he was in MS, he didn't take it well.
Also, know that is your baby. There will be no different in it and the child you carry in your tummy. I don't mean to be tacky, but if you think it will be, you may not be ready or right for adoption. If you ever treat the raising of the child different because of the adoption (either better or worse) there will be huge emotional issues with both children!
I can't imagine the screening process that adoption agencies have in place but I know it is long and tedious. Which is excellent to weed out the people who aren't right for what ever reason.
Good luck with what ever you decide.