Narnia and PotofGold make excellent points.
I am slightly worried by some of your wording, I am sorry if that sounds offensive. We all, adopters, choose children of ages that would suit and work better in our family - we did.
We had a 9 year old birth dd and we were joined by a 3 year old adopted son. For us this was an ideal age difference and has worked well, they are now 11 (soon to be 12) and 6. So it is not your desire for an older child I am slightly unsure about but the "I just would like him to be okay. I'd hate to go through with everything, then he starts to change and then need specialist 1-1 help. I don't know how to describe it."
You can't describe it, can you try so we can understand where you are coming from?
And...
"I'd like a child that would fit well into it, and would be able to be a part of our family - do the things we do, etc. We have obviously been matched due to things he likes as well and it just all seems so perfect."
I think it is totally normal to want a child who will fit into the family, and like what you like etc. I think to some extent we all want that (or most of us do).
To some degree I think (especially with a younger child) that does happen. They develop tastes and enjoy what you enjoy. This has happened to some degree with ds who likes the TV programmes dd likes, enjoys some of the same foods etc and doing some of the same things (e.g. swimming, having not swum a lot).
I think to some degree this kind of fitting in might be more likely with a younger child.
Wanting to adopt an older child is pretty admirable but I do think you need your eyes very much open to what this will mean. They may well like the same things you all like now but this may change and you may need to be prepared for this. (As we all are with our adopted kids, of course.)
Clearly the biggest alarm bells (for me) is the fact that you do not know why the child returned to their home country and your use of the words "Birth parents use to send him back to original country to be used for various things (not 100% what as have little info on it (so does SS)" are pretty scary!
I totally get you will not be able to say this on an open forum (please do not give away too much info!) but in your shoes I would need to find out as much as possible and make your peace with what you do not know, if it is going to work out.
Good luck. 