I think it's normal to get nervous at this stage. Even if it's something that you've wanted for a long time, it probably always seemed abstract and "someday." I assume that you're nervous because of the surgery itself and what the recovery may entail for J, and that you are permanently heading into "no kids" territory. And yes...your friends' daughter may be cute but remember...it's different when they're your own. Ha! All of those negatives that you listed would be YOURS to deal with. No thanks, right? :)
Once the whole thing is done and J's recovered, I imagine you'll feel more like a big weight has been lifted and the anxiety will disappear. I wish you guys the best of luck for the procedure next week!