I'll second the combination of Optech and E1 strap. I have both on my ancient 1D and they make the size of the unit a lot more manageable. Have to say that I looked at a 5D recently, primarily for the improvement in outright image quality. The problem was the handling. In comparison to a 1, the 5 may be lighter, but it's far less balanced and nothing like as comfortable to use. In addition the AF and build quality are so much better on the 1 (even my ancient unit).
So I'm sure the mk3 will be a seriously good piece of kit.
Keep us posted how you get on.