Yes, the thought of birth defects is scary .... and the risk does increase after age 40. I have no words of wisdom here. It scares me, too. Is this something you could see a genetic counselor about? Find out if you have any risk factors besides age?
I don't know anything about the adoption process - have you checked out that forum here at mdc? I'd imagine that the adjustment a child has to make to a new sibling - be it a natural sibling or an adopted sibling, is probably the same. Just my guess though.
This whole TTC #2 requires such a leap of faith - every morning I'm sure we're doing the right thing, but every night, right before we actually start the um, mechanics, of TTC #2, I freak myself right out. If I start thinking about everything that might go wrong, or how tired I'll be in the beginning even if everything goes right, I really start vacillating. I just keep thinking about my "future" family....
edited to add: re-reading my post, it appears I haven't been very helpful...at least I bumped you back to the top, right? Hoping someone else has some better advice for you.....