If you're doing this for a business thing, then well, yeah, that isn't a smart way to go about it. Chasing the newest for what's mostly not that much additional utility. Going more towards every other generation than generational successors is a better model.
The benefits of the D850 over the 810>autofocus
Can be make or break depending on what's being shot>4k video
This is actually kind of a push because you can get a D810 plus one of the various m4/3 or crop bodies that do 4k and video far better than any Nikon system camera ever dreamed of>higher resolution
This is one of those ehhh things in that the biggest benefit to most shooters is being able to crop in (as most aren't printing large enough to warrant the 34 MP of the D810 much less the 40something of the D850), but there's a downside too. The 30-40 meg RAW files that the D810 produces make a lot of computers cry. If it's been a few years since you've updated your computer, an 810's raws will require you to upgrade your comp and 850's will make you go even farther to get decent performance while doing post.
If you're really wanting high resolution images, you might as well go for the Pentax 645z, especially if you want something that'll keep its value, but I'd personally aim for something like the D810 then when I had the extra money to be able to afford the D850, I'd buy another lens, get lighting gear, or set up some shots to sell. Just buying a slightly better body that mostly doesn't do anything my other body does just isn't worth sinking twice the value of my current body into.