Kodi uses buffering. On a slow network connection when you play a file it won't start right away, it buffers some of the file. Usually it is pretty smart about how much to buffer to get a smooth playback, but it isn't always perfect. The issue you were having is DLNA streaming is different than what Kodi would be with a HTPC. DLNA is very sensitive to network bandwidth and in my experience suffers more from slower network connections like wireless. Straight file sharing through windows file sharing works a lot better over wifi than DLNA, and that is what you would be using with Kodi and a HTPC.
Kodi.
Not right away. Worst case is if 4k starts to overload the computer just throw a more powerful video card in it. Since Kodi(or any good media player) uses GPU hardware acceleration to playback the video, simply adding a more powerful GPU will allow you to continue to play higher and higher resolution movies.
Any of the 500Mbps ones should work. I use the
trendnet micro adapters in my house.
Going over that length I'd say there is definitely some input lag introduced. There wouldn't be a quality loss due to HDMI being digital, but I'd definitely say there would be some input lag.
You know, another alternative for all of this might be the
NZXT DOKO. It might just take care of everything you need.