I think these are the things I'd consider:
Age gap - mine are 5.5 years apart so I don't consider your 'age gap' as an issue, but maybe you do. I will say that the age difference was more 'difficult' in during some periods of their lives. An 8 year old doesn't want their 3 year old sibling wanting to involve themselves in the 10 year old's play with his mates. And a 16 year old certainly doesn't want an 11 year old tagging along to the movies or whatever. But that age gap difference 'narrows' as they get older and my two are thick as thieves since DS2 got to his early teens.
Relationship - Is your relationship stable? Is your DH an equal co-parent?
Finances - Are you and DH financially stable? Would you be financially able to parent alone if 'something' should happen? Would your financial plans for retirement head off the rails with the added expenses of another child? At 45, if you have to 'divert' some of your income to child costs, you wouldn't have the 'luxury' of years of making up those lost savings or pension contributions the way younger parents do.
Health - How is your general health and energy/stamina? Remember those early years? We often forget how physically taxing a baby, then a toddler can be. And remember that as we get older (especially if we're 'older' to begin with) we do lose some of our 'pep', and often some of our patience.
Longevity/Genetics - OK, I know this one is a real crapshoot and 'life happens', but still worth thinking about. Giving birth at 45, you want to have faith that you'll still be alive and kicking to get this baby grown, educated, and independent. Are your and DH's family 65+ year olds still active and full of energy (and patience)? Is there a history of early onset dementia or Alzheimers?
Retirement - DH and I retired in our mid-50s (I know we were lucky), with our sons grown and independent. We wouldn't have been able to do that if we'd had a 10 year old. But even if someone wants to retire in their early 60s it would be difficult with a child still in education, or if you were paying Uni fees. There's nothing wrong with thinking about the impact of being an older parent on one's own future plans.
I know a lot of what I'm saying is purely practical. And having a baby isn't always based on 'practicalities'. But I think my points are worth thinking about. And personally, I don't think that 'giving a child a sibling' is really a good reason to have a baby, if that's the main reason for doing it. Maybe you'll decide to go ahead, and I wish you all life's blessings. Maybe you'll decide to be happy and focus on the child you have, and in that case I wish you peace and joy with your decision.