I have installed markdown-preview-mode-20171121.2323 from MELPA (not the stable...) and this installs also markdown-mode-20180124.138 and other packages. All this to be used with my Win64 builds of Emacs master using MSYS2/MinGW64.
I tried to test this with a simple README.md file but both "M-x markdown-preview-mode" and Emacs menu/Markdown/Preview % Export/Preview (C-c C-cp) fail with Error running timer: (user-error "markdown failed with exit code 1") Notice, I have also installed the "markdown" package in MSYS2, pacman -S markdown ... and (getenv "PATH") prints "C:\\msys64\\usr\\bin;C:\\msys64\\mingw64\\bin;[...]". I tried also adding this to the init.el (setq markdown-command "C:/msys64/usr/bin/markdown") but same results.. Ciao, Angelo.