Hmmm, I think you need to take advise from people who actually HAVE a 50 1.2 mounted to a 5D MK3
I have one mounted to mine 99% of the time.
Fast focusing (yes on the MK3 it's as fast the 1.8 )
Sharp and kinda ordinary from F/2 down
Beautiful and dreamy wide open - something the 1.4 and 1.8 WILL NOT ACHIEVE
Very hard to get shots in perfect focus wide open
I have the 50mm F/1.8 and this is a great lens optically for the $100 price tag.
Get this or the F/1.4 and you will have a great walk around lens. Lighter and better for learning composition than the 24-105 + they work in lower light.
I hated the 24-105 F/4 that came with my kit and sold it instantly.
The 1.2 is a great lens.
Wide open it will give you something the others cannot.
I don't know about the Zeiss 50mm F/1.4, but there are issues with the focussing screens being fixed on the MK3.
It's supposedly excellent for video on the MK3 (because you use LiveView) and at $700 is half the price of the 1.2.
You are going to have a steep learning curve with either before you get a lot of keepers.
I bought a Fuji X-Pro 1 + 35mm F/1.4 for my wife, but I've been "learning how to use it" before handing it off to her
This camera is capable of excellent results - some say it beats the 5D MK2 in IQ.
I think the MK3 + 50mm F/1.2 produces better results.
The Fuji 35mm (53mm equivalent) is sharp wide open and the result is you cannot achieve the dreamy bokeh of the 1.2.
So, it depends what you want.
The 1.2 can be stopped down and look like any other 50mm.
The other 50's cannot be opened up to look like the 1.2