Hi, ok, I'm going to be brutally honest here. If you have a birth child already - don't do it! Even if your 'adopted child' is several years younger than birth child -and- you can be absolutely sure that birth parents have absolutely no interest. You could be bringing absolute chaos into your birth childs life in a few years.
An adopted child who has any memories of birth family will naturally be curious about birth family, and thanks to social media - birth parents (lets be honest, it'll only be birth mum) will be easily found by the adopted child, and then you will have to deal with that. Adoption agencies have now recognised that adoption in its traditional sense can no longer be achieved, so are following something called 'open adoption' where contact between birth parents and the adopted child is promoted far beyond the annual 'letter box' contact - this means that your parenting will be constantly undermined by people who have a completely different set of morals and interests to you.
We have both adopted and fostered, and I can say categorically that fostered children who have regular contact with birth family are constantly unsettled and disregulated, and that will affect other children in your family. Especially when such contact is made into a lovely day, presents will be given and there will be a little party to make birth parents feel better about their neglect/abuse - but the child will get a message that these people arent so bad after all.
I suggest if you feel strongly that you want to extend your family, offer short-term respite care to foster families - you will get a taste of how things are when Social Workers undermine the parents and put the needs of an abusive/neglectful parent equal to the child.
If a couple of years of short-term fostering doesnt put you off, then maybe consider adoption - but dont enter into it naively, because the social workers will paint a rosy picture and only tell you half the childs story.