I just had our fourth child, but he's 14 years younger than his closest sibling so it's not really the same situation at all. It hasn't been a breeze because I'm juggling a full time career and a baby (and don't believe in day care so he's home all day while I'm working) but having older children does help with a little one. So probably not a lot of help there for you except in the sense that you mention you're the bread winner and have professional aspirations. It won't be easy, unless you're willing to compromise a lot; especially on how much time you get to spend with the baby.
My sister has three children under the age of 4 and has said emphatically that's it. No more. Going from 2 to 3 was difficult in juggling them all at once, though she's getting a handle on it now. Because you've got one that's 7 it will be easier, I think, because the seven year old is more self-sufficient than very young ones and can actually be a help to you with a baby.
On the c-section front. Both my natural born children were c-section births: one in 1987, one this year. The risks are the same as you get older, but the older you are the more risk that your uterus will rupture because it's naturally getting weaker *and* it's been cut twice now. There's an increased risk of scar tissue and problems that result from that (endometriosis can be a result of c-sections as I learned 21 years ago) and there's the wear and tear on your body from yet another c-section (it's unlikely they'll let you deliver naturally after 2 c-sections) and of course the typical risks associated with anesthesia/spinals/epidurals.
There is also the increasing risk of Downs and other genetic problems with the baby due to your age and the general strain on your body from pregnancy because of age. The risk of miscarriage is much higher as well.
I would never trade my surprise 4th child, but I would not have chosen to have a child at nearly 40 years old. It's more dangerous to the mother and the child.
L.