Read "The German Generals" by B.H.L. Hart. Their take was that the Russian Army and Armament was greatly improved by war's end. And the Germans thought the Russians had the best tank of any Army by War's end.
Something similar was true of the Soviet air force - it got much better.
The Germans ate the Soviet air force up in 41 and 42, and held even with it in 43. However, read about some of the Soviet offensives in 44. The Soviets successfully executed massed dive-bomber and fighter-bomber attacks. Their fighters also successfully kept any German planes out of the air. They also did do some quasi-strategic bombing you don't hear much about in the west. They hit cities in the Baltic states and parts of what was then eastern Germany with medium bombers. I don't think it's a fair assumption that they would have posed no challenge for the RAF+USAAF team. I think the RAF+USAAF were stronger, and would have won eventually, but there would have been a few months with no clear air superiority during which our ground forces would have struggled against the Soviet armor.
Lend-Lease: difficult to say if the Soviets would have lost without it. We gave them tinned foods like spam, rail locomotives, refined petroleum products like aviation fuel, some artillery, a few tanks they didn't like, and loads and loads of trucks. It was probably the trucks that made the biggest difference.
However, they made the vast majority of their tanks (T-34s, KVs, "Joseph Stalins", SU-86 assault guns, etc) themselves. By the fall of 1942 they put these out at a rate of about 2000 a month, a rate which they sustained until the end of the war. They also built some mind-boggling amount of artillery themselves. All of that output probably would have happened without lend-lease. Same with aircraft: they were turning out tens of thousands a year.
Without all the trucks, canned food, extra locomotives, refined oil products etc. they probably would not have been able to move that mass of stuff forward as effectively. The Germans would have gotten more opportunities to regroup and counter-attack. Likely the Soviets don't win without lend-lease. But do they lose? Too close to call. Maybe stalemate.
Or maybe Hitler orders some other crazy grandiose offensive and the Germans end up losing whatever forces they would have saved up... and the Soviets just kind of slowly and ponderously lumber into Berlin.
Also, in regards to letting Patton go for Moscow in 1945: by that point the Soviets were making trucks and larger quantities of refined oil products themselves, and they already had all the other stuff we had given them. So, the end of lend-lease wouldn't have hurt them as much.
Best case scenario would have been if we actually had atom bombs by May 1945, and not just two of them - but several. In that scenario, we might have used the atom bombs to destroy the Soviet factories in the Urals. The Soviets would then have been left with just the forces they had in Europe - and those would have only lasted a few months without reinforcements.
I'm not sure under which scenario the human cost was greater. In the scenario of a war against the USSR, millions would have died. On the other hand, millions of people led crappy lives under Soviet rule. However, others managed to do OK, or to escape to the West. Which is worse? I know a lot of us would say "give me liberty or give me death", but that's our choice - don't know if I want to speak for the people of eastern Europe and Russia in saying "you would've been better dead than red"... especially as their descendants now have a chance.