The main purpose of Stalin's actions in East Europe at the end of the Second World War was simply a continuation of Russian policies since the 16th century, to expand as much as possible in all directions. Russia had gained and then lost control of Lithuania, Estonia and Latvia, and of course had regained them during the war. Russia had invaded and seized parts of Poland more than once, invaded Germany in 1914, and invaded Poland and the Balkan states (intending to invade Germany again) during the 1918-1919 period, just after the Revolution. British and French troops helped the Poles and Balkans drive them out.
Stalin did not seek a buffer between the USSR and the West, he wished to dominate the Eastern European countries. What he set up were simply puppet governments, which had no more autonomy than those countries had enjoyed under the Nazis. In some cases, such as Romania, less. His goal for Eastern Europe was to set up Communist governments which would be controlled by the Soviet state, with Soviet generals controlling their militaries and Soviet "political advisers" controlling their governments. He had the provisional government in Poland in 1945 arrested and executed, replacing them with puppets, etc. To him, control of Eastern Europe meant control of their resources, industries and populations as a means of continuing to expand the control of the USSR and the "world revolution."
The ending of World War II's conflicts in Europe posed a rather interesting situation. How would the reorganization of Europe be defined? For his part, Stalin believed that he wanted to create a configuration that would prevent, once and for all, another Western threat to Russia. In preventing this, Stalin created a block of nations that had featured governments that were puppets of his Soviet government. Up and down this eastern corridor of nations, Stalin's "pupils" and tactics had helped to define governments that were extensions of his own Soviet government, complete with their own purges such as in Hungary and Bulgaria. At the same time, this helped to foster another one of his goals, that being of Soviet Supremacy. As Stalin was able to control these nations in Eastern Europe, he also ensured a consolidation of his own power in these nations, helping to establish him as a dominant world leader until his death in 1953.
After World War II, the Soviet Union had occupied much of Eastern Europe, including Poland, Czechoslovakia, Hungary, Romania, Bulgaria and East Germany. Stalin had two main goals in doing this:
1) Establish a "buffer zone" between the Soviet Union and Germany. Russia had been invaded by Germany twice in the last 30 years, and Stalin wanted some protection between him and Germany for the future, and for Germany to remain divided.
2) He wanted to spread communism to new governments. He established communist strangleholds on government on each of the Eastern European states he took over, and they stayed that way for nearly 50 years.