Had he not attacked first, Western Europe would have gone communist by the 1960s; had he not attacked then — and remember all choices were disadvantageous and led to the same grave — Russia would have attacked Germany pretty soon, perhaps by 1944, and would have had overwhelming strength ratio: Russia would dictate to Germany and the nazi empire would roll up into submission to the new master. Britain would have had to come to some accommodation to Europe, but with most socialists and liberals urging a soviet Britain as the British Empire broke away, they too would have eventually succumbed.
In the real world, the USSR would not have won on the Eastern Front had it not been for the truly massive supplies given by the USA — but had they been the attackers later facing an exhausted Germany which had exhausted it’s plunder economy ( obviously a victorious nazi Germany not facing any immediate threat would have stabilized the European economy to some efficiency eventually, possibly much as at present ) the fighting would be on, and devastating to, Polish and German soil, not as in actuality devastating Russia, and they would have been invincible.
WWII Germany has often been compared to a lightweight fighting three or four heavyweights simultaneously — in manpower, but most particularly in economic power — but when you are in the middle of an alley and you are going to get a beating anyway it’s probably best to strike first at the nearest than stand and wait for them all to punch at once.
There might not have been a second front, and even had there been, British and American troops if they landed in France would not have the werewithal to fight a soviet empire stretching from Mongolia to the Rhine.
There would have been a Cold War between the USSR and America, but neither would be strong enough to attack the other, until one or the other’s economy collapsed.
The only upside, for people who didn’t like communism, is that nearly every intellectual back then who idealistically urged socialism/communism, would have had the entire rationale of their existence torn away by success — nothing more left to strive for — and then each would have been carted away to a slave-camp by the soviet masters. On general principles.