#5 is basically correct about Marxism, but I will take it a step further: since Marxism, Communism, and Socialism are three related but very different ideaologies, each must have its own little explanation.
Marxism: "From each according to their ability, to each according to their need." Since all work is done for the collective, and since no one has the right to take more than another and risk it on speculation, there can be no foreign investment or interest in any way. This is the pure form, idealized and Utopian, with no one claiming elite status and no drains because everyone is contributing.
Communism: generally a form of governmental dictatorship or "Lords and Serfs" deal. The government (which does not exist in Pure Marxism) takes all the resources and doles them out as it sees fit. In this case, all work is done for the government, which has the power to speculate if it chooses. Foreign investments would be possible, but unpopular because of the money it removes from the internal economy (remember, these are all intending to be a zero-sum game).
Socialism: removal of the private sector in favor of the public sector, or the governmentalization of the entire economy. While Communism has a working class and a ruling class, Socialism has neither, instead trying to make everyone part of the governmental class. In theory, this would ensure equality, but it falls prey to the same foibles; no one wants others to have more (money, power, control) and so the structure fails. Foreign investments would be key to keeping money flowing, but again they would likely fail in the long-term.
Historical studies of M/C/S societies are useful here.