My DS was about 5 when he joined our famiy, so I don't have direct experience with adopting a child the age you're considering. However, I would advise you to read everything you can get your hands on regarding older child adoption. There are also several older child adoption yahoo groups that would be helpful to you.
Integrating DS into our family was one of the hardest things I've ever done. I was so glad that I knew what to expect beforehand. And I was so glad to know that other parents had faced similar challenges and overcome them. I was also relieved to know that other parents had similar feelings about their adopted older children (OMG, this is the worst mistake I've ever made! and when am I going to start liking this kid, nevermind loving him???)
My DS has been with our family for 8.5 months, and it's been a bumpy ride. We appear to be through the worst of it, thank goodness. I can't underestimate the importance of being prepared. I knew it was going to be hard, I didn't know it was going to be as hard as it was- but at least I was prepared for a challenge.