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."