Just solved it. I had had the same issue for days and got fed up with re-downloading the game in a never-ending cycle. Here it goes: 
1. download the game 
2. install the game and ignore the error messages 
3. close Steam 
4. re-start your PC 
5. start Steam 
6. play the game 
I fixed this by a complete mistake. I had to go somewhere else and shut down my PC (a rig quite similar to yours, BTW) for the duration. After returning and firing up the engine, everything worked flawlessly.