The Finns did indeed do very well against the Russians during the Winter War, but only some of the factors which were involved were applicable to an eventual German invasion of the USSR. Finland, which I’ve heard described as “a country whose topography consists almost entirely of obstacles to military operations,” i.e. lakes and dense forests, is an ideal terrain for defenders, especially in harsh winter conditions. The Finns made these natural advantages even more effective by capitalizing on them correctly, notably by using fast-moving ski troops in winter camouflage smocks. Western Russia’s terrain, by contrast, is relatively flat and open (which favours the attacker), but is enormous in size (which allows the defender to mount a defense in depth, which is a good way to deal with a mechanized attack).
It’s quite correct that Hitler was very impressed by the performance of the Finns against the Russians – or, to put it another way, was encouraged by how badly the Russians did against the Finns – but unfortunately for him the Russians similarly took note of how badly they’d done in Finland and undertook reforms to correct these deficiencies in the Red Army. By mid-1941, the Red Army and its leadership had improved significantly. Moreover, Hitler seems to have overlooked an important element of the Winter War: the fact that the Finns did end up losing once the Russians brought the full weight of their enormous numbers to bear, without regard to the casualties they took in the process…which in many ways (but not exclusively) is how the Russians ended up beating Germany in the 1942-1945 period. The “not exclusively” part refers to the fact the the Soviet Union did make qualitative improvements in its fighting ability, not just quantitative one. A good example is the development of the T-34: when the Whermacht starting encountering it for the first time around late 1941, the Germans – who prided themselves in their armoured forces and technical expertise – were shocked to discover that a nation of (in their view) backward Slavic peasants had produced the best medium tank in the world. (It was especially humiliating for them to note that the Russians had developed a diesel engine powerful enough to drive a tank, something which the Germans had up to then not been able to accomplish.)
As Caesar Seriona correctly points out, Stalin did build a buffer zone on the western border of the USSR; on the Global 1940 map, the territories in question are called Vyborg, Baltic States, Eastern Poland (with German collusion) and Bessarabia. It should be noted, however, that Hitler similarly built himself a springboard for an invasion of the USSR by annexing or conquering or forming an alliance several territories of his own: Czechoslovkia, Hungary, Romania and Western Poland (with Russian collusion).