bibbetybobbityboo (great name).
I will answer your points with my own thoughts, not to advise you what to do but just to share where I am coming from. My dd was 18 months old when I was told we would not have any more children biologically. I didn't believe the doctors and years of fertility treatment followed. We were told DD had to be 5 before we adopted but now it is different, younger birth children are sometimes OK. We ended up looking into adoption when dd was 7, starting the process when she was almost 8 and adopting when she was 9.
It is not always easy, my dd (who can be a handful) can be jealous and so can ds! But they love each other, care for each other and are in many ways like regular sister and brother. My son has few issues, so far, he was three when placed with us, not far off 4. There is a big age gap, almost 6 years. I believe this can be very helpful and is advised. But do read up.
Re your points....
I read on adoption forums though, that adopting with a birth child is a terrible idea, that they will ruin her childhood, that she will resent it etc. and I just don't know what to do.
I expect for some it does, some it doesn't and some are in the middle. But remember people usually post threads when things are going badly, or at least I think start threads when things are not going well. If things are going well you may reply to other threads but are probably less likely to start threads or discuss, 'normal day today, all well!'
Re Is it really a guarantee that adopting will disrupt our lives immeasurably? Am I being selfish I wanting this and in fact risking everything?
There are risks, no one can tell you there are not, but there are also situations where children placed can live relatively 'normal' lives. But more than that the children may well thrive and your dd may rise to any challenges.
I also think that in the past families might have either tried to treat adopted children very differently (either well or badly) and set up resentments etc or tried to just treat everyone the same, and not recognised attachment issues and other problems and suitable help may not have been available.
You are not selfish!
Re ...I do worry about putting dd at risk from that point of view.
You would be able to talk to social workers and family finders about what you feel you could and could not handle, you and your dh would need to know your own limitations and be on the same page. I also feel a big age gap does enable your child to be the bigger one so as children their is more chance your child will not feel bullied or whatever by the newcomer. But you may see some sad aspects of it as your child may be the one doing the bullying! They may resent the new child, but that could (and often does) happen with any new sibling! My poor sis tells some tales about me!
You are also right that being a baby doesn't make them 'immune' to issues or problems or trauma, and if they are pre-verbal they may not be able to talk about stuff or understand it. I think there are a lot of factors, attachment disorder, lots of moves, alcohol in vitro etc etc but some children may have a very rough start in life and still be able to bond and connect well. You would need to continue reading (here is a great place) and trust your own judgement once you are informed.
Good luck.
Wrote all this and then read Whatutalkinboutwillis post and should have just said.... What Whatutalkinboutwillis said!