The good news is that you are already doing daycare so you have the routines, and know how part of it down already. Just as you would transition a new child into care you will transition your baby into care. My own were 4 months and almost 2 when I started daycare and had a week off with my third and fourth. There are 10 years between my first and last which was a surprise too.

With so many advances in medical care I wouldn't be too worried about the age situation. It does feel a little daunting after you have gotten used to the idea that you are done with the baby stuff only to realize you shouldn't have gotten rid of it all. But with your own older kids it can actually be a lot of fun for them. My kids were great with the baby and she really was a family project that wouldn't have been possible if they were also all still small.