Was the whole "let's help rebuild Europe after WWII" thing just to create a buffer zone against the USSR?

You're referring to the Marshall Plan. It was to create governments who share our values regarding freedom and democracy, so that they could become stable prosperous allies and markets. USSR had already created a buffer, known as the Iron Curtain.

"Buffer" for whom?

Those who lived in what became called "Western Europe" would not have appreciated that view of them.

Partially. Germany in particular