Wow. This is a tough one. The World Health Organization recommends three years between kids as the optimal birth spacing. Obviously, here in America, we have access to better nutrition and medical care than many places; still, there are risks to both mom and baby when pregnancies occur too close together that are purely biological. Besides the worry about the c-section scar, you also have to think about whether your body is up to growing another baby over 10 months. Too- closely spaced children is a big risk factor for miscarriage and pre-term birth-- something that you are already at increased risk of being near 40. Many women are still anemic a 6-12 months after child birth. And, obviously, the risks of having a child with Down's Syndrome goes from 1:139 to 1:109 between 39 and 40-- can you handle a special needs child when you have another so young at home? These are HUGE and difficult questions and I think you need to find a doctor you trust and have a very serious conversation with them about wether your body is ready to handle this vs. the risk of waiting.
(this is to say nothing about whether YOU and your husband are ready to handle it... that is a totally different question you guys will have to decide together.) In the end, get informed and make a decision you are comfortable with-- there are no guarantees in life whatever you do, only probabilities.