Hi there,
I did this exact same switch a few months ago. My work is mostly portraiture as well and I am loving the Credo 40 tonnes. For my work, it's an upgrade from the D800 in every possible way.
To answer your questions, yes; skintones are amazing out of the box, but process the files only in Capture One. No other converter does justice to the files.
With Leaf shutter lenses, you can get upto 1/1600 sync with strobes. I haven't gone that high, but I get a comfortable 1/500s with my Pocketwizards. In a studio, it's not a big deal as your flash duration is more critical. Since you're using Bron, that's not a problem at all.
Resolution: It's a lot more than the 4MP difference would suggest. The Credo resolves a ton more details than the D800. You can sharpen the D800 files yes, but it's not the same.
Don't worry about the crop. It's actually a blessing in disguise for portrait work as you get much better pixel density. The viewfinder is still miles better than the nikon's.
Regarding the lens, you can do full body work with the 80mm, but if you want something close to your 135 Zeiss, I would recommend the 150mm LS.
p.s. Of course, all the usual MF caveats apply. Shooting process is slower, AF is basic, etc. etc. Buy boy, are the results worth it!
Enough talk. Here are some pictures: