I'm not entirely sure about the AMD side of things but I'm running 4K now and I've been using nvidia GPUs and currently have a dell P2715Q.
Generally DVI and VGA are nearly dead, they're getting phased out. We're in the arkward situation where support for HDMI is significantly better on TVs and entertainment devices, while DP wipes the floor with everything else on PCs. If you must standardise, use DP for PCs. If you decide to connect your console to your monitor (I don't judge!)
On the monitor side:
SST >>> MST. Its simpler, you get the full 60 hz refresh rate, and amusingly,
its better supported by hardware in most cases. You'll need DP 1.1a or better but meh, as you'll see that's not hard
Use mini DP or DP in, not HDMI - the standard for HDMI on PCs is an older standard, monitors may support the same standard and will only do 30 hz. In fact, if you must standardise, DP first, and DP for your best displays.
If you're doing photo editing, getting a properly colour calibrated screen a good idea.
If you can daisy chain, it'll cut your refresh rate in half. Not too recommended.
On the computer side:
I've gotten 4K output on a 660. In some cases I've managed to game badly on 4K, and the output wasn't the bottleneck, the ability to render the game was. I believe the 650 is the minimum GPU from nvidia for 4K (they have a handy list here) and anything better than that should handle it fine. 4th and 5th gen (and better) Intel video should be able to handle at least one 4K display. I can't find an equivilent chart or resource for AMD.
Something worth considering is the modern UHD display is only 8 megapixels. While its challenging to render graphics, and you might need power to render video, it should probably be fairly simple to build a system that can handle photo editing