the latest R* dlc was simply amazing the yachts , the offices , that bitch in the reception , the safes and especially the Tv Game. the mods said that they will create a simmilar experience as the gta:online.
It will not directly include the dlc, like the yachts and the offices. Or any of the heists etc. However it will make this possible to be created yourself.
It depends on the game version. If we use the game version that came with the yacht DLC then yes, the models are included but the GTA:O feature of buying a yacht and playing missions on it are not supported. So whatever DLC it is, only the models and new content will be in the game just not the missions and features that come with it. You can script it in, though.