Good luck with your search.
I sure hope monitor tech has a good year. I don't see it so much myself.
I have an UHD 31" display and a 34" 21:9 2560x1080 secondary, both 60Hz. And the higher resolution is just way more valuable for me. You don't need to run games at UHD, but for the games that benefit from higher res, Civ 6 comes to mind, I just couldn't go back. Shooters and racing (thats what, 30%-ish combined of the market? ) benefits from higher refresh compared to higher resolution from strategy,sim,RP, adventure and MMOs (50-something% IIRC combined). So yes, it's not something most people will agree on as it is generally divided between dedicated gamer types. SO I know what I use wont work for others, not claiming it to be the best for anyone else.
But what sucks about 4K, it's not about the content, there is plenty of worthwhile content and workspace benefit for running a 4K monitor in ... 2014 even(I was an early adopter), let alone today... is the lack of higher refresh rates. I know some people convince themselves that content isn't there, but wow. Going back to 1440p is a huge hit, even with the better hardware drivers pushing 144Hz RR. We need the best of both worlds soon, and for whatever reason, I just don't see 2017 having a good push for 4K 100+Hz monitors. Maybe a couple come out, and they wind up being $3000, but fuck that. I did that a few times (buying monitors more expensive than the computer) and it NEVER pays off to be the early adopter of display tech. It could be that I'm missing fanboy rhetoric somewhere too, where 4K is better with one company and 2K is better with another, therefore obvious splits and spits, but I couldn't tell you since IDGAF about fanboy arguments.
My whole point is, I've been waiting since early 2014 for a 4k 30+" monitor with 120hz+ with vesa mounts and hardware standards, but nothing doing other than a few ridiculous niche monitors with piss latency or uneven brightness. You might be waiting for a few years to find your monitor from a decent vendor in spec. Widen that budget and get what you really want, because $300, even on sale, will be a real stretch for the foreseeable future.