Sean,
I honestly do not think that I want a parent who is "not up for parenting a child" to have one (using that as the basis for not adopting a special needs child). The reason for this is simple. You do not know what kind of child you will have biologically, adoption wise, etc. Special needs can manifest at any point in a person's life. For example, I was "perfect" at birth. My symptoms of muscle disease did not manifest until I was two. I walked until I was 16, and so forth. I think a lot of people have this mindset that it is okay to have a kid with special needs if it comes from their bodies (and even then some will reject their child), but not to adopt a child with special needs.
You cannot minimize the risks of genetic disorders (i.e. Muscular Dystrophy, Cystic fibrosis, SMA). You cannot predict Autism, Bi Polar, Schizophrenia, Cancer, etc. Yeah, you can find out if your child has Downs Syndrome while in the uterus, and many choose to abort the baby due to the Down's (which makes me extremely sad), but most disorders do not work this way. Nobody in my family had the genetic disorder I have. No one. There was absolutely no sign I would have it, even though it is autosomal recessive and I received it thanks to a faulty gene both my parents had. A lot of kids who have SMA have families who do not know because both parents have to have the faulty gene to pass the disorder on and even then it is a 1 in 4 chance it will be passed.
If you have a child - adoption, biologically, etc. you need to realize things might not go according to plan. Your child could be autistic, have physical or learning disabilities, develop a mental disorder upon their impending teen years when most manifest. Saying you could never handle a kid "like that" is an indicator someone isn't ready to have kids, at all.
Yes, adoption is expensive, but I also look at the growing population. We're already pillaging the planet's resources endlessly. Why bring more kids into a world like this when there are so many already?