The reality is that Stalin probably suspected right up until the last minute that any evidence of a German attack was created by the British to try to fool the Soviets to fight the Germans too. At the time the British were pretty much on their own and desperate to get some real allies against the Germans and for them the two best options would be the Soviets and the Americans... you can bet your ass they were at the Soviets night and day telling them of the Germans plans to invade the Soviet Union.
The facts were that before the German invasion of Poland the Soviets asked Britain for an alliance with Poland against Germany and neither were interested and in fact they feared the Soviets more than they feared the Germans.
Nothing in their history would suggest any reason for Stalin to trust Britain at all.
The Soviet Military was in a state of transition from one model to a different model based on what they learned from combat in spain and in Finland.
Also they didn't want to put their forces on a war footing in case that provoked the Germans to attack in a preemptive self defence type manner fearing the Soviets were preparing to attack them.
And they had enormous defensive lines in the region that were dismantled because of various changes in planning that would have made the German invasion much harder.
There were things that would not have changed... most of their aircraft, while potent and powerful when they first entered service in the mid 1930s were now obsolete.. the Polikarpov I-15 and I-16 are obvious examples, and they had them in enormous numbers, though fortunately because they were out on the airfields instead of hidden away most were destroyed on the ground so more pilots survived that might otherwise have been killed in the air in inferior fighters... while they also had a lot of armour, again much of it was light tanks like T-26s which were very ordinary and were not designed for the modern tactics the German tanks used. The German tanks of the time didn't have better guns or better armour, what they had was tanks designed to operate in teams where the commander didn't have to load and fire the gun he could just be the commander and find targets and threats.
Of course they had good platforms too... Yak-1 and MiG-3 and T-34 and KV-1 but in small numbers to make a difference.
Another problem was a lack of anti armour ammunition... having a big powerful gun able to penetrate the armour of any enemy vehicle means nothing if you only have HE rounds which wont penetrate armour...