EM 5 is the base version. EM 2016/2017/20 have extra content (somewhat DLC like). All Versions work with BFENG as long as the game is updated to the game-bersion 4.2.0. Emergency 5/2016/2017 doesnt get sold anymore so you only can get some old keys or EM20. Modding EM 5 isnt that particular harder then modding for EM4. It just has a bigger hurdle to get into and needs longer time to work into. Especially with the unfortunate lack of english documentation.