I would be concerned - as I am sure OP is- that if dd needed to be supported into a parttime job, how will she cope with fulltime motherhood. She sounds like she has no experience of independent adult life (not so unusual for 18!) and was still very much a child in the home who required her parents' support.
Any forward planning has to assume that boyfriend will not be around for long. He might, of course, but I don't think him sticking around for long can be planned on.
Ideally she should start working straight away, if her health allows. She should have at least some experience of working life and something to reference in future job hunting. Even if only for a few months.
I fear this is not the OP supporting her daughter so much as the OP essentially having another child, and that the vast bulk of the emotional, practical and financial aspects of having a child wiĺl fall on OP.
That's a big shock, and a lot to take on.
What are dd's future employment or education plans? Has she felt well enough to do any work towards that, even at home?
I'd say, for next steps: dd finds a job and possibly learns to drive, if she is well enough and you live in an area where driving is important. Meet with boyfriend's family and get a sense of the practical and financial support they can offer.
All the best, OP. The baby is coming, that's a fact, and now it's a question of how to help the baby and dd into the best life possible, while doing the same for dd2 and hopefully yourself. Wishing you lots of strength.