I'd be looking at the practicalities in this situation...
Is your ex involved with your current child? In other words, do you and current partner have child free time over the weekend etc? If so, invisage a time where your eldest has to go to their fathers, and your youngest is obviously with you and dp 24/7. Will this be a bug adjustment to your current set up?
Do you have much family support? Hands on grandparents/aunties & uncles, who will be involved and happy to help out with two children with completely different needs and wants due to the age gap will certainly make life easier in the future, and without that support I'd probably not be so keen to have another baby. I know friends with big age gaps and the grandparents will only look after the eldest, and think they're helping out by having one of the kids overnight, but actually it makes things trickier as they're left with a grumpy bored younger child who doesn't understand why they haven't been able to go too, and the couple still doesn't get any kid free time.
Don't expect the kids to get on, your eldest is at an age where a baby is not going to just slot into their life, and will make things harder for all, how do you think they will find this? Having to clear away their legos, and small toys when baby is crawling, having to have their routine suddenly dictated by baby naps, and nappy changes. It's a bug adjustment for the existing child as well as the parents, and that is harder with a slightly older child who has known life as an only for a longer time. Also, obviously only being biologically yours, will this impact the relationship your child has with their step father? It's hard for the existing child to not be pushed out when a baby comes along, especially when they become part of a blended family where they aren't related to both parents of their new sibling.
Days out, trying to please both will ultimately become harder, and you may find you end up doing things separately to accommodate both their needs.
All things I'd be considering in your situation.