The Marshall Plan, and NATO, were intended to contain Soviet expansion rather than communism per se. Yugoslavia was a Communist country, but after the split between Tito and Stalin, the US had friendly relations with Yugoslavia.

As to why containing Soviet expansion was regarded as a vital interest of the US: remember that this was in the aftermath of WWII. The US had just participated in an all-out war against Nazi Germany, which could perhaps have been prevented if the Western powers (primarily France and Britain, the US having disbanded its army after WWI) had taken a stronger position during the early stages of Nazi expansion (Austria, Czechoslovakia). The US feared a Soviet takeover of all of Europe, which would threaten the US.

George Kennan was a central figure in the early Cold War, including the development of the Marshall Plan. His 1946 Long Telegram from Moscow provided a detailed picture of the Soviet leadership and its worldview.

More generally, Kennan noted that US security depends on the balance of power in Europe and Asia. In the Walgreen Lectures (published in 1951 as American Diplomacy 1900-1950):

Today, standing at the end rather than the beginning of this half-century, some of us see certain fundamental elements on which we suspect that American security has rested. We can see that our security has been dependent throughout much of our history on the position of Britain; that Canada, in particular, has been a useful and indispensable hostage to good relations between our country and British Empire; and that Britain's position, in turn, has depended on the maintenance of a balance of power on the European Continent. Thus it was essential to us, as it was to Britain, that no single Continental land power should come to dominate the entire Eurasian land mass. Our interest has lain rather in the maintenance of some sort of stable balance among the powers of the interior, in order that none of them should effect the subjugation of the others, conquer the seafaring fringes of the land mass, become a great sea power as well as land power, shatter the position of England, and enter--as in these circumstances it certainly would--on an overseas expansion hostile to ourselves and supported by the immense resources of the interior of Europe and Asia.

Or as Jefferson put it: The enduring interest of the US lies in preventing the entire force of Europe from being wielded by a single hand. That's true whether we're talking about Napoleon, Hitler, or Stalin.

In short, the US feared that history would repeat itself, with Stalin playing the role of Hitler. The US sought to strengthen Western Europe to prevent it from falling into Soviet control through sheer exhaustion -- in 1947, Western Europe had been shattered by the war, had failed to recover as expected, and was in danger of imminent economic collapse.

Mark Danner, writing in 1997:

A half-century ago, Germany found itself in a state of utter devastation, its people clawing through ruins and brambles in search of scraps of food; France and Britain were financially and spiritually exhausted; and in the occupied states to the east, the soldiers of the Red Army crouched menacingly, the spearhead of a nation that had arisen triumphant from the war and saw before it now no power that could possibly oppose it.

If you're not familiar with the early Cold War, Louis Halle's The Cold War as History is very readable. Or if you want a look at a short primary document, here's PPS/1, the initial policy recommendations on the Marshall Plan from Kennan and his staff.

I think you inadvertently touched on an often overlooked point when people say today that we are in a "New Cold War" with Russia, because there is a fundamental difference between then and now:

The Soviet Union offered an ostensibly legitimate economic alternative to capitalism- socialism.

Leading experts on the Soviet Union like Moshe Lewin described Soviet socialism as a "camouflage for Bolshevik ideologies" and Stalin's reign as an "agrarian despotism", and repeatedly refers to the Soviet government as a confusing, monolithic, oftentimes hypocritical bureaucracy.(Can read more about this in his book Russia/USSR/Russia: the Drive and Drift of a Superstate)

So besides these totalitarian, "socialist"(socialism as outlined by Marx was never achieved, instead it sprung up in pre-capitalist countries) bureaucracies on the border of Western Europe, why else would the West want to keep them away?

From 1945 to 1973 capitalism in the West hit its "Golden Age", where per capita income in Europe grew at a rate of 4.1 percent per year between 1950 and 1973. In the U.S. the rate was 2.5 percent, which was still unprecedented. West Germany specifically was 5.0 percent.

None of this was an accident, but it brings us to another point in your questions- where do you draw the line between economics and politics? I think it'd be more useful to ask: when should the state get involved, and when should it not? There's no clear cut answer for this. Many conservatives want less government, whereas many liberals(people who identify politically as liberal today, not liberal economics) want more government involvement.

You might have heard of laissez-faire economics, which is essentially an economic system where government involvement is kept to a bare minimum. Laissez-faire was the slogan of more old-timey liberals, which is why it's important when we're saying the word "liberal" to make note of who exactly we're talking about.

The Great Depression showed us the limits of laissez-faire capitalism and spurned more government involvement and regulations(see: FDR's First and Second New Deal). But how much government is enough? How much is too much? How much is not enough?

That's debatable, and those debates emerge in political discussions. Should the U.S. have universal healthcare and raise taxes on citizens to achieve this? What about free university? Social security? How can we keep jobs in our country? How much should the government invest in certain technologies? There isn't any hard line between politics and economics here.

I also never used the word "communist" or "communism" here since I'm inclined to agree with Professor Lewin that the word was nothing more than a type of camouflage that the Soviet bureaucrats liked to throw around.

The numbers pertaining to per capita income in the U.S. and Western Europe come from Ha-Joon Chang's book titled Economics: The User's Guide.

I hope this answered some of your questions at least, and I'd welcome more input pertaining specifically to the Marshall Plan. You can also search this sub for threads pertaining to the Red Scare and McCarthyism, as these topics address communist/socialist paranoia in America.