I'm running KDE Plasma 5.4.3 on Debian stretch/sid. I have kwin_wayland 5.4.3 and sddm 0.13.0.
I only installed the base system and then ran
apt-get install kde-plasma-desktop/testing sddm/testing (and all updates & upgrades afterwards and before).
It kinda works - there is a nice login screen and it can create windows; However none of them have any window decorations (e.g. the minimize/maximize/close buttons are missing) which I miss dearly.
My .xsession_errors file is here
There is an older question with my problem, but it is for KDM which isn't used in the new version.