Just a few details concerning some points I read earlier.
The gap of the front barrel
Yes, dust can get in the lens, probably from this gap. I own this lens for about two years now, and I had to have it clean twice. It's no big deal, my local professional store did it with ionized air, by delicately taking the screw off on the mount. Dust seems to go between the two rear glass.
This is the lens I use the most, and the only one not weather sealed I have, so... I'm guessing a mark III would be, like all recent L-lens upgrade.
Don't compare the 50 f/1.2L to the 85 f/1.2L II. The 50L is a first generation, with a lot of CA. the 85L II is much more controled and the Lightroom profile takes care of what's left.
I use the 85L on a 1D Mark IV (1.3x) and it's fine. I really think it's not noticeable on a APS-C (1.6x) sensor body. 85mm x 1.3 gives a nice 110mm, almost like old times 105mm portrait lens.
Yes, you need to use this lens for a while to get used to it. But once you get a hold of it, it'll become your go to portrait lens. The background are creamy and contrast perfect for B&W. This lens is really made to make your subject stand out. It's worth the investment.