They're desperate to find families for hard to place (older) children - MOST agencies will not consider an out of birth order adoption because it's a high risk adoption unless it's kinship or foster family adoption (already you can see that in exceptional circumstances the general rules get bent). But I have to say, this is the first time I've heard of any agency saying that it's okay for a 'stranger' adoption.
I honestly don't think you'll find anyone here with any experience unless they are a kinship or foster care adopter - it really is that rare and unusual.
I would personally not recommend it, because it IS a high risk adoption. And I have older children, I know first hand both how rewarding and successful an older child adoption can be, AND how bloody difficult parenting kids like mine can be. I also have the experince of raising an older child with emotional issues alongside a younger child (who was adopted later on). That's also simultanoeusly somehing I do not regeret AND very very hard. My younger DS HAS been affected by his older sister's emotional issues, he HAS been negavtively impacted by it. That's an inescapable fact, much as I don't regert my decision to bring him into the family. Are you prepared to accept any negative impact on your BC?
It happens a lot more in America, and some adoptions are very succcessful, others not so. And they very much acknowledge that it's more likely to go wrong if it's out of birth order. But it's not inevitably going to end badly, they can be very succesful. So I don't think an out of birth order adoption is doomeed to end badly, not at all, but I still don't think it's worth the risk unless you have a very exceptional case
First off - are you absolutely 100% comfortable with not having another baby? Especially if you've been mentally preparing for one. That's something to think hard about. Matching is also about the needs and wants of the prospective family - don't budge if there's a part of you that really wants another baby
Secondly - Older children are much more likely to have emotional, social, developmental or behavioural issues that are immediately apparent. Especially emotional and attachment based issues. Much more likely to have been neglected and abused. More likely to have significant ties to their birth family (that doesn't necessarily mean contact, but it can mean strong emotions and memories). You need to seriously think about what needs and issues you could think about taking on, given you have a younger child too
An older child is going to need a LOT of attention, care and supervision. But because your BC is young, they also need a lot of that. Can you manage that? Can you avoid falling into the trap of thinking that just because your BC is more physically dependent on you, that means they need more attention and supervision than an older child? Because older children need a huge amount of it, every bit as much. They frequently need parenting as a younger child, because they frequently have a younger emotional age than their biological age. A 6 year old, even if they are doing well in school etc, could very easily have the emotional age of a 3 year old, and need the same amount of nurture and the same kind of parenting as a 3 year old. Then can you basically parent two 3 year olds at the same time and treat them like each other?
An older child might come out with aggressive behaviour, or mean behaviour, jealousy, many issues. What if your 3 year old gets in the way of an agressive tantrum? 8 year olds can throw objects at other people with quite a lot of force (experience! Book/train track to the head = not fun. 3 year old getting hit in the head with a book or train track or just plain old slapping and kicking - even worse. Again, experience, from after i adopted DS). Can you treat your children the same then, even in the very early days when you don't yet love your new child? After all, you've just made a lifelong commitment to the older child, whatever they do or say
What if your 3 year old is copying bad behaviour? Older children can be controlling and manipulative, not in a malicious way or even consciously, but because those are the behaviour patterns that are ingrained from their early years. A 3 year old can learn that, and be impacted by it. Teenage years? Maybe your older AC struggles a lot, but then your younger BC is right in the middle of it
These are all just things to think about, and it's all from experience. The most important bit to absolutely accept, is that older children are often emotionally younger by a few years than where they should be, and need parenting accordingly. This isn't an issue if that adopted child is the youngest in the family, but bear in mind that you're unlikely to get a reasonable emotional age gap between the two, if you adopt a 6 or 7 year old.
I'm not trying to be doom and gloom by the way - and I'm very positive about adopting older children in general - but I'm trying to be realistic as well. You really need to think carefully
Thirdly - What support are you going to get from social services? They want to meet government targets and place children. But if they want to support a more risky adoption, much as it might work out, they better be prepared to acknowledge that it's more risky and they better be prepared to give you good support.