I don't do the weddings on my own I normally have back up camera's from the team but I do like working with two camera's
The lens ranges I plan on getting step by step
Are 70-200 and 200 -500
28-300
And a few primes the thing I don't like about the d800 is im not a massive fan of using flash even in low light so tend to rely alot on the iso, and yea I am worried about the weight of the gear, but that I can always work out later, I am sort of leaning towards the 5d3 but the 6d I wouldn't need to sell my 580 I guess, ive held and played with all 4 cameras I must say I don't massively like the handling of the d600 but at the price and the features its hard to brush of especially that depth of field is nice. The only thing that puts me of the 6d is the auto focus in movie and normal and I don't like diving into the menu to change the iso and I didn't feel it was much better then my 580 if anything i though it seemed better at solid colours it wasn't too good on transparent colours. But im sure there are ways to play with the settings. The only the I really like about the 5d3 is that auto focus
And about 10 odd years ago I had my x300 pack up on at a wedding but I didn't realise it until after because it was the film days and the shutter was only half opening and it sounded fine until you process the film. Lucky I wasn't the main photographer there I was just backup