If you want to do weddings, the 70-200 is a must. But it doesn't necessarily have to be Nikon. Sigma and Tamron have a very equivalent quality lenses to offer at a much lower price. By buying either third party lens, I would then get a D600. This is the best bang for the buck in the full frame range. I know the 750 is better, but at almost double the price. If you consider that there will always be newer bodies that will be out in around 2 years, buying one pre-owned is usually the best bargain.
So, you could get both a great FX body and a great 70-200 and you'd be in business for the same price as just a D750 with not 70-200.
So, you could get both a great FX body and a great 70-200 and you'd be in business for the same price as just a D750 with not 70-200.