Hypothetically, you need to decide whether to concentrate on full frame or crop format. You appear to be looking at a mixture of both (hypothetically of course

) by introducing the D700. Sure you can use them together, but it's not ideal. And the 18-50 is a crop format lens, which overlaps substantially with the 24-70 which is full frame :shrug: And SB600s are a bit short on power for a wedding photographer (can you use them as Masters in wireless flash? Not certain of that).
If I was kitting up for weddings and stuff, and thank the Lord I'm not, even hypothetically, it would be full frame probably with a D700 for its low light performance. Or Canon 5D2. That would be at the heart and I'd build a system of Nikon lenses around it. I wouldn't compromise on crop format with Sigmas as I know I'd want to upgrade pretty soon if business went well.
You should be able to do pretty much any wedding with a D700 and 24-70 2.8, plus a big flash. Have another camera and kit lens ready as back up and hope you never need it. Nothing else is essential to get a very good job done, but a 70-200 and a fast prime would fit in nicely as and when budget allows.
You will get better results with one really capable camera and lens that you know how to use well, and concentrating on the subject rather than the equipment, than you will with two different format cameras and swapping lenses all the time, mainly concentrating on the kit.
But since this is all hypothetical, it doesn't really matter