It's not adoption, it's permanent fostercare which is a different thing - I've done both.
Don't be afraid to say no if that is what your gut is telling you. But I have done it - in fact I was originally approved to do respite care only and ended up switching to permanent after just 6 months.
If you do decide to go ahead then do think clearly now - this is the time when the LA need you more than you need them. So if you are going to go ahead with this, think about all the things you may need for this child in the future and get an agreement on the provision of those things now.
Do you need respite?
How will the contact work, will you have to be supervising it or will someone else need to be involved in that, how flexible will it be, will it prevent you from doing things like having family holidays together (that would be a dealbreaker for me; contact HAS to fit around our family life, not our family life be squeezed around contact)? What happens if relationships between the family and yourself break down, who will do the transporting, etc.?
Is the child accomodated under sec20 or is there a care order in place? (you don't have to answer any of these questions by the way, I'm listing the ones I ask, not expecting you to respond to them) If Sec 20 then how will you handle living with a child whose parents have the right to undermine everything you try to do? If care order then what are the reasons and is your family safe? What are the birth parents' views on the need for fostercare? Why is the child separated from siblings and are there implications in that for other children the child may meet?
Is the child likely to be having problems at school, does the child access CAMHS, is the LA going to put pressure on you to take out SGO or RO?
Does the child have challenging behaviour, does anyone else think the child has challenging behaviour (having had one child where SW said "oh no problems" but it then transpired 9 year old had a habit of being extremely violent and had possibly been abused by a visitor to the family house). Can you meet with school staff, with whoever is presently caring for the chld, with the child's doctor and anyone else significant to the child?
Again - please don't feel you need to answer these questions (and I do understand confidentiality and I'm not trying to undermine it), just these are the questions I ask before accepting a placement myself.
My long term placement came up quite suddenly like this, and I had to make a decision based just on the Form E (you should see that form E before making decisions; there's a lot of info not on it but there's quite a bit that is on it too) before I got to meet the child, child moved in 4 weeks later and we had a very happy life together (although it wasn't without its problems).