I have the ZE (Canon mount) version of this exact lens, and my friend has the Nikon ZF.2 version, so am in a good position to advise. It has both pro's and con's, and they must be considered carefully as this is no small outlay of change to purchase!
Firstly, this lens is amazingly sharp when you get it right ...
I say this because I've used it to create some portrait and macro images that I'm extremely happy with. However, wide open it can be frustratingly difficult to maintain focus if you're using it handheld. At f/2 and up close, your depth of field is less than a gnats whisker and so much as breathing will send your subject out of focus. I often shoot it at around f/8 just because of the depth of field at 100mm...
I have developed over the past couple of years a somewhat interesting way of using it, by taking a deep breath and holding it, bringing my elbows in close, and tensing my whole body so as to minimise movement.
Of course, that doesn't help with subject movement. I was actually out with it today at lunchtime taking advantage of the nice weather we're having in the UK at the moment (a rarity!), and with just a gentle breeze, the flowers I was shooting were just swaying a little which made it tricky to get focus and I had to wait for the gaps in the breeze.
I think your concerns about the manual focussing -- other than the fact it can be frustrating if used wide open -- are mostly unfounded. The focus throw is long, giving you lots of fine control, especially at short distances from the subject. It's also buttery smooth. Smoother than any of my other Canon lenses including the L ones. It just feels right.
You will get to know your focus confirmation dot very well. On the Nikon D800 (an AMAZING camera and perfectly suited to the Zeiss), you will also have two little arrows pointing which way to turn the focus ring (a great help, and I wish Canon had that feature!).
If you can control your subject, and mount the camera on a tripod, then much of this problem goes away. I have done this for product shots from time to time and it is fantastic at picking out detail in anything you point it at.
I have also never had a problem with the lack of weather sealing on this lens. I take care of it of course...but after 2 years of ownership it is perfectly okay with no evidence of dust or anything inside it.
A feature the Nikon ZF.2 version has over the ZE also, is a manual aperture control ring, if you so wish to make use of it. If not, turn it to f/22 (I think) and it takes the aperture as set on the camera itself.
Also I love the 'look' that pictures taken with this lens have. Especially when you have fine detail such as the texture of a leaf, or hair on someones head.
Pros of this lens are:
- Blisteringly sharp.
- Beautifully made - it feels absolutely solid.
- The focus throw is buttery smooth.
- The resolution of this lens is outstanding. I would say it far exceeds my Canon lenses, even my EF 24mm f/1.4L II USM.
- Will make the most of the high resolution sensor in the D800.
Cons of this lens are:
- Sometimes frustrating to achieve focus hand-held.
- No auto-focus.
- Women may hate it as it will show up every blemish on their skin. You may need to artificially soften the skin in post production if doing portraits with it. (Not sure if this is a con or not really!)
If you are interested you can see more of my images taken with the ZE version of this lens at my Flickr account as well as some with the ZF.2 (on a D90) which I borrowed when I was trying to decide if I wanted it or not.
All that said, I can also vouch for the Nikkor AF-S 85mm f/1.4 G as you mention. It is also stonkingly sharp and would be great for portrature, however the minimum focus distance isn't as good as the Zeiss so as you say, you wouldn't be able to get "up close" with it.