When the German army was 80 to 90% still horse drawn at the start of Barborossa then I think they were unprepared, and as Barborossa started late their was no provision for a war in the Russian winter. That really screwed the Germans up, no winter provisions of wintrer oils, uniforms and still too many horses plus a logistics chain that was both long and behind the front increasingly insecure.Realistically, in order to achieve what Hitler wanted (the conquest of "Russia", the extermination of the Slavic population, and it's replacement with "Aryans"), Germany needed to defeat Poland and incorporate it into the new expanded German state.
Hitler didn't really describe the ultimate fate of Poland in Mein Kampf, other than to describe it as an obstacle that must be destroyed before achieving his ultimate objective of the conquest of "Russia". But I don't think that Hitler really differentiated between Poland and Russia with respect to their ultimate fates. I suspect he simply thought in broader terms of "the east", and geographic terms such as "Poland" or "Russia" would be relegated to the history books once the new German territories were conquered and settled. There had been no Poland for centuries before WWI, and there was to be no Poland after WWII.
What made German plans ultimately fail was that they didn't have the ability to conduct a protracted war. If they had built up large strategic stockpiles of food, oil, and other raw materials, they might have been in a much different position during the invasion of the Soviet Union. There were a lot of things they needed to do to prepare for the war, but hadn't had ready in 1939.
If I recall correctly, the German navy were told to prepare for a war which was to start in 1943, and their construction plan was based around that. If that reflects the real overall plan, then an additional four years of preparation might have helped the Germans somewhat, although their enemies would also have had more time to prepare.
There is a line of argument that the Germans were approaching bankruptcy by 1939 and so were forced into action prematurely. If this is true, then that might explain some of the lack of preparation, although I don't think the Germans realised just how unprepared they were.