If I had the money/time to spend raising a kid, I would totally do it. My mom was adopted so paying it forward only seems right, plus it would be literally life changing for the kid.
Would have to be a slightly older kid, maybe a preteen or so? I watched a movie as a kid about a main character who lived in an orphanage, and I can always remember a scene where they were watching younger kids (toddlers or so) get adopted and the older ones getting passed over, so that would be a big 'rule' for me.