In my experience, no, most babies don't have severe issues (although I would say that what people consider to be a severe issue varies wildly). Some babies do have physical issues, some babies will have developmental delays. Because nearly all waiting children have been removed from their birth parents by the courts (not willingly given up by a young mum), most babies available for adoption have background factors that need careful consideration. They might have been exposed to drugs/alcohol in utero (this might cause problems later), there might be a family history of mental illness, learning disabilities, drug misuse etc, and some babies may have been in an abusive/nelgectful environment in their first weeks of life before being removed
The likelihood of any adopted baby going on to develop SN or additional issues is higher than the likelihood of a child who hasn't been in care. IME how affected a child will be by their experiences, depends mostly upon a combination of how much trauma they have been through, and their own personality/genetics. The more trauma they have been through, the more likely they will have problems as a result of it, and now it is believed that a persons resilience has a large genetic component etc. Age is not as important as either of those two factors IMHO, although the older a child is, the more trauma they have probably been through, so the more likely they would be to have various issues.
Do you think your children would cope with concurrent planning? It might involve contact with birth parents up to 4/5 times a week, SW's coming and going, and there's a possibility, however small, that the birth parents might get it together and get their baby back. I would be wary of it if you have a child with SN.
I would say that the likelihood of a baby adopted internationally developing issues is exactly the same as for a UK baby, and in many countries you have exactly the same background issues - for instance, I believe in Russia alcohol abuse is a big problem and many many of the children in orphanages were exposed to alcohol in utero. In addition, if the baby has been in an orphanage, there are additional risk factors - lack of stimulation or adequate care might create issues, developmental delays are really common etc.
Of course not every adopted child will go on to develop SN, some escape unscathed by their experiences, but predicting which children will develop additional needs and which won't is very difficult if not impossible in many cases, so to adopt you do need to be okay with the uncertainty.