Oh, goodness OP, perfectly reasonable to discuss this.
Totally understandable that your DW wants her ‘own’ child. And that you would want to do that with her.
Also totally understandable that your children would have feelings / insecurities around it. Even the 19 yo. Will the new baby supplant them, will you still feel like their Mum, will you be prioritising another woman’s baby, how will they feel about a sibling that lives with both parents while their family was split, etc etc. These are all valid feelings to have even if the reasonable rational answer is ‘of course you have the right to be happy to have a second family with your DW’.
Can you take them off on a day trip and gently discuss how they feel?
I think it is reasonable to consider whether you have the means to support more through Uni etc as your 15 yo has still to go.
Also, though I wish you the very best of luck with whatever route forward you choose, do explore all feelings around pg by donor. It did cause unexpected emotional reactions in both partners in my friends family, and their Dc too. Not saying this can’t be navigated, obviously it can and many people have been made very happy, but it can bring unexpected things to think about.
You are all family, your D.C., your DW, you love them all, of course you are sensitive to everyone’s feelings.