I'm running a Dell UP2715K which is the 27" 5K monitor they used to sell. I think it has the same panel as the 5K iMacs. Don't worry about the resolution at 4K at this size either, as long as you are running Win 10 the scaling is excellent, it was at the time I got it better than was available on my friends iMac. I run mine at 175% for normal windows programs, it usually recommends 200%, which on a monitor that is 219 PPI is actually about right for comparison to most non HiDPI 27" screens. Screen real estate might be at about the same size, since many monitors are running about 110 PPI or so, but the windows scaling is running at the full resolution, and is really taking advantage of it when doing things like font smoothing. It's why I can take advantage of running at 175%.
As far as working on images goes I would really like a monitor that ran at 300PPI, so I could get 1:1 scaling on my print output. If you use Adobe programs you will need to move to Lr6 as a minimum, and I think CC for Ps for HiDPI support. Adobe don't use the normal Windows screen drawing APIs, so you can't use the OS scaling options. Some open source programs also have this issue. Viewing images at 1:1 view is still relevant, you soon get used to the new output resolution. My one niggle with Lr is that in Loupe view you have to be in Fit as the viewing ratio to use the scroll wheel on a normal mouse. When I first had the monitor the then current Lr version would limit fit to screen to a maximum of 1:1, which was nice. Any image that would fit inside the viewing window would be size limited. Anything bigger would be shrunk down to fit. Now there is no limit to the magnification of Fit to View, an image that is 288 pixels high will be interpolated to fill up to 2880 px. Where in the past I have keepers that might be cropped as far as 1024px on the short edge if being used on screen only, if you scroll to one of those you end up thinking what was I thinking keeping that. A quick change to 1:1 view usually works. I do now routinely check images at 2:1 so that I will see them at the same size as those using a normal resolution screen.
When it comes to colour spaces, I use the Dell monitor management software that comes with the monitor. I think it works with most of the UltraSharp monitors. This allows you to change the colour space in use on the monitor on the fly. I usually have it set to auto. For Lr and other programs that use it I have it set to AdobeRGB. Lr uses AdobeRGB in all but the Develop module, so it is a good choice for Lr. When I swap to a browser though I switch to sRGB. Although browsers are now mostly colour managed, many images have no colour info at all. So defaulting the browser to work in sRGB seems like a good idea. Properly tagged wide gamut images will be correctly converted to sRGB, and sRGB ought to be the closest match to any images with no tags. It really works for me, but I was caught out by Premier, I thought it was supposed to be colour managed, so had it set up to use P3, a common wide gamut specification for video work. It turns out that Premier is only capable of working internally at sRGB, and only outputs sRGB to the screen.
I would be looking for the highest resolution screen I could get at any size. Resolutions up to 3880px wide 4K are not an issue, but above that you can hit some snags. The higher resolutions often still need dual connections, because although many screens claim to support Displayport 3, they can really only manage DP2 speeds reliably. You will usually require an actual GPU to support this, the minimum that I know of is an Nvidia GTX 960, I think you have to go with a 1060 if looking for a new card. I have no idea on ATI cards. Wide Gamut is good too, but I would be looking for a monitor that came with management software that allows you to swap spaces on the fly.
Alan
