For family portraits and any high res applications the D800 is hard to beat. The D810 is about 4 db lower in level, measurable but hardly noticeable in sound level. But the two have a different sound and for most people the sound seems softer, less mechanical. For studio, shutter is not an issue.
You are making me rethink my decision to pass on that nearly new refurbished D800 . . . Best for portraits you say . . . hmmmmm
The ART 50 and the 85 1.8G are your go to lenses, both very competent. If you don't have flash controllers, folding soft-boxes, grids, snoots, multiple flashes or strobes, C stand, light stands maybe background, a large silver/gold reflector, gels, and other very reasonably priced items that is where the best return on investment is You can get nice 9 foot stands for $25/ea, 80x80c collapsing soft-boxes for $50 from GODOX(great for location shooting folds downs to a small cash the size of a woman's purse, a strip box with grid for about $100, a flash controller for $50, transceivers for $49 each, and some dumb non-TTL flash units for $80-90 each or used SB-28s for $25, various clamps and adapters. That would be a good portable studio so you can shoot in their homes or outdoors. Backgrounds should be simple and white is the most flexible, easy to use for seamless, High Key, and easy to tint in post processing and much preferred for product or catalog sessions. Pure white background works best for web catalogs If you go with studio strobes, ever the low cost Alien Bees, like the 800 watt, it is less portable and power source becomes a problem outdoors. A couple speedlights can generate enough light to balance a setting sun and are surprisingly powerful. I use 6 with Yongnuo YN-622n transceivers and YN-622 TX LCD display controller that have proven to be reliable and cheap. I hardly ever use my strobes now.
I would encourage you to invest just a few hundred first in versatile lighting
My current lighting gear includes the Godox AD-360 barebulb flash and two AD-860 lithium strobes, both controllable by the Godox X1 trigger allowing full control of light output from the top of the camera. I also have two Godox receiver units that work with my D600, D700 and an old Vivitar Auto-Thyristor 2800-D and sort of with my Metz 58. And a reasonable collection of softboxes and umbrellas and stands. Considering proper strobes - not Ailen Bees as they are not sold in Canada and importing them is expensive, rather buy from a dealer I can go to, either Godox or Elinchrom set up, but leaning more towards battery pack system like the AD-600 by Godox ($1,000 CDN). I will be spending more on lights - maybe portable strobes, maybe continuous, might be studio strobe if I can be convinced it is portable enough....
and if you still want to buy a camera, the D800 (and D810) are the best portrait cameras Nikon has ever made.