I'm a big fan of getting things started regardless of where you are due to the length of time it takes to get stuff done. You can move in the lengthy application process!
I've lived in a flat and a house, and I enjoyed aspects of both. Flats can be sociable, and compact and as you say, allow for a greater level of freedom financially. Living in a flat could be great in many ways- compact living means that you are able to be super close and responsive to your child's needs, you don't need stair gates, there are lots of barriers between them and an open road, you likely have plenty of amenities quite close by, parking can be secure and there is probably good disabled access if it is a block of flats with a lift for buggies and stuff like that. As you say, it frees up your income to allow for more flexible working or other opportunities. That may be very necessary, and you might not know that until your child has been placed for a little while.
However, I would say that a flat possibly limits the type of child/ren you could offer a place to, or might in a SW eyes.
We have two very active busy young boys and even the idea of being in a flat with them for extended period of time makes me want to cry. It's all very well and good saying you can take them outside or keep them busy with clubs, but that means you being outside with them to supervise, or have them attend classes. It is very public and can be hard to maintain. It can be hard to go to classes if your child has boundless energy combined with a developing confrontational/defiant/PDA profile who laughs in the face of any teacher who tries to make him do a crab walk at tae kwon do or frog legs at swimming. They can't cope with the level of classes and social interaction that would be required to burn off the amount of energy they have outside school. In the house they are destructive and throw stuff, jump on and off the couch and bash stuff around. It is absolutely exhausting having them inside for long periods of time.
Our garden is tiny but has been a godsend. Sometimes you want to be able to just let them in the garden to bash rocks together whilst you get the washing up done without having to negotiate their behaviour at a park, or worrying they might start a fight, or do a runner. It allows for messy play and easy potty training and noise outside is less than inside.
I would also note that moving house may be incredibly stressful and traumatic for a child, and I would say that if you told a sw you planned to move house early on in placement that might present some questions because the child would need stability for as long as possible. We are nearly 2 years into placement and our kids are only just coming to terms with our house and local area as "their" home, and the idea of us moving them now seems incredibly traumatic unless we actually genuinely had to. Moving jobs and losing a parent are apparently two of the most stressful things you can do, and your kid will have done both of those things at least twice in their short lives, so I would be trying really hard if possible not to make it a third or more for a very long time after placement if I could possibly help it.
When we do eventually move, we want to go for a detached house because our neighbours no longer speak to us after several middle of the night screaming fits, loud temper tantrums at all hours and the not infrequent sound of me bellowing something absolutely ridiculous across the house about not using the patio doors as cannon ball practice. If we had to worry about several sets of neighbors up and down I think I might faint. It's just another layer of stress and anxiety that can be hard to manage and may impede your relationship with them. Before they stopped talking to us, I was constantly apologising because I felt bad that their kids had exams and mine were playing drums on the wall and wailing themselves to sleep at 3am because they needed to show they had some control over some aspect of their lives. However, the more neighbors you have, the worse I would imagine that feels!
I'm sure many people have adopted just fine in flats- don't be put off by random horror stories about social workers freaking out over balconies and what not, because you can do stuff to make them safe. For me, things like flat safety can be accomodated and mitigated for, it is about the lifestyle of parenting potentially tricky kids. Of course, people who have bio kids were probably OK, but were they neuro diverse kids with special needs? I'm not saying your adopted kids will be necessarily be ND, but the percentages and likelihood are quite high, so I would proceed accordingly. Stuff like being able to have a mini trampoline, messy sensory play, a contained outdoor space that you can control, all those things are helpful when thinking about the provisions you can make for an ND kid. No, they aren't completely necessary, but if I had a choice to work an amount that was appropriate for them AND give them the space they needed, I would try and give them that.
I hope this makes sense- I'm definitely biased because I currently live in a house with two little boys who just love constantly climbing, being messy and shouting, and having that bit more space and a private outdoor area means that we can let them get it out their system, not having to constantly take them out from the moment they wake up to the moment their heads hit the pillow, which would be the alternative! But I can see the other side of it, and would say that it had some real positives of living in a flat and having that financial freedom too. Plus, my situation probably wouldnt be the same if I only had one kid, or had a quiet and retiring child. So i suppose I would say, start the process, think carefully about what life might be like in the worst case scenario and prepare from there. If nothing else, having these discussions with your partner and social worker will show you have really considered the needs of any potential child.