1 - This display is going to look identical to a 5k iMac, or a 27inch Apple thunderbolt display, the text will be very readable.
2 - Windows handles scaling very differently to OSX, and whilst most OSX appliacations have been updated for retina displays, this isn't common practice on Windows yet (so some apps are either tiny or VERY blurry using 4k on Windows)
3 - No, it's only input is Thunderbolt 3, from what I've read, there isn't anyway to convert to that yet, so you're reliant on having a machine with TB3 onboard (although I'm not even sure this monitor has drivers for Windows either)
There's a great article here;
https://bjango.com/articles/macexternaldisplays/
That talks about the optimal size screen for 4k/5k etc, for me personally, 4k (1080p screen space in Retina mode) looks too big on a 27inch device, and native with 4k, everything is way too small. The 27inch 5k is perfect for screen space and text clarity in my opinion.