One important thing to consider is whether or not your existing computer and software can handle the larger files from a D810. You may need to budget for upgrading either or both of those. Also, how are you going to store and archive the large files--do you have enough storage space? When I went from 10mp to 24mp I was shocked at how much more hard drive space I went through.
Another thought: TTL flash is great, but for really interesting work off camera lighting is often better. You could compromise with something like a Quantum Q-Flash that can work in TTL and is also versatile off camera. Or, get a lower model Nikon flash for TTL and a decent monolight. I'd rather have competent strobe equipment available than every lens in Nikon's catalog.
Also, ignore the side argument about the best 50. After a point, "better" just doesn't matter in the real world. The best 50 (or best of any lens) almost certainly won't make you any more money (which is paramount if you want to be and stay a pro) than one infinitesimally less good.