The things you really need to think about as a single adopter are
finance - how will you fund your adoption leave, what if you need to go part time at work - can you afford it? Children generally are expensive. Childcare can be hideously costly -its one of the reasons I adopted a school age child. Can you afford it
support - its hard work, you can't do it on your own. Support doesnt have to mean family, it could be friends, neighbours. It does mean a bit more than a spot of baby sitting, What if you get stuck at work, are ill, what if the child is ill. what about school holidays, teacher training days, snow days etc. Whose shoulder are you going to cry on, who will you vent to, who will get some shopping in for you. You will need to provide at least three referees.
work - teaching may give you long holidays but day to day isnt the most flexible job. This year we had several snow days, my son's secondary school was closed, my office wasnt, we have three local primaries, two closed, one stayed open. Your child is likely to have some additional needs - how will you handle appointments. I work three days a week, I manage to get most of our appointments on my non working days but it isnt always possible. I also have flexible working - I can go in late, have a long lunch, finish early, as long as I work my 90 hours over the month it doesnt matter.
I adopted over 6 years ago, I did have some issues as a single adopter - but that was more with an individual social worker and I got there in the end. Many SWs do prefer couples for their children and I do agree that some children require the energy of two people. I know lots of single adopters though.
If your brother is living with you, do you both realise that he will need to be assessed alongside you. How supportive is he really of your plans. You say your brother often needs your help - what does that really mean? Does he work ?
At the moment I think your priority would be to move back to the UK, get settled and build your support. You need to think seriosuly about the idea of shuttling back and forth between Spain and the UK. Adopted children really need stablity, particularly in the early years. You need to think about language and the impact of a different culture on an insecure child who will probably have addtional needs. Not to say you couldnt move back in the future, I do know adopters who have returned to their home nation - but these were to English speaking countries.
I have the same difficulties as most adopters, getting support, funding for therapy, finding the right schools etc etc. Its just harder on your own - the emotional weight of decision making is all yours. Being the only income earner, ying to maintain work and looking after your child - this is a big issue for me at the moment. We are six years in, my son is 14 he is doing well, but there are issues causing me great stress, I'm not looking forward to GCSEs, - three phone calls this morning from him about a piece of holiday homework. I don't have a career anymore - I have a job which pays me what I need to earn.
Its just a tough old job, I love my son, he is an amazing and brave young man. He has exceeded everyone's expectations. I don't regret it for a second - but it is hard