Hitler was aware of the Soviet military purges of 1938, however he was not sure that the military power of the Red Army would have been so much weaker, until the Winter War. This led to two things: Hitler being adamant that he could win in a short campaign, and Stalin making huge changes to the Red Army´s equipment and tactics.

So without the Winter War which was following the Ribbentrop-Molotov pact Hitler would not be willing to attack east first, I think.

__________________