I’m both a social worker and adoptive parent.
The assessment process will explore your current living circumstances, current and previous relationships including the relationship with both your parents, siblings you might have etc. They will want to know why you’re not in contact with your dad, how that came about and whether you’re in a position to offer the new baby a stable, loving home.
Things you really need to think about include your financial position, are you financially secure and will you be able to take leave from any job to care for the baby at least for the 6-9 months, possibly much longer if the little one does indeed have additional needs. Are they talking foster placement (eg paid foster care) or kinship care (which would usually be the case in a family situation but doesn’t come with the same financial support). If you decide to adopt, it’s unlikely there would be any ongoing financial support other than child benefit and DLA if applicable.
What are your current and future plans - you don’t say how old you are but I imagine you might have had plans for birth children which may need to be reconsidered. If you’re not in a relationship a new baby might really get in the way if you did want a partner - how would you manage that?
Given this isn’t something you were thinking about, and has come at you very quickly it’s important to use your heart and your head. I totally hear your concerns for the baby but you also need to think about the impact on you not least so that you can plan for the future.
A newborn isn’t likely to bounce between foster carers, it’s not unusual for babies with quite complex needs to be adopted so potentially having Down’s syndrome wouldn’t necessarily be a barrier to adoption.
Most people who come to adoption have had a long time to think about it and prepared, and the assessment process is designed to make sure those people are realistic, have considered the impact on them and understand the challenges of parenting through adoption. You’re coming to this fresh, with a lot of emotion around your dad and this new baby (understandably). Try to take a step back, think through your options and the changes and challenges coming your way if you do proceed.