You can decrease the number of particles emitting from the CityBase object.  You can also decrease the number of samples cycles is using to render.  Likewise if you decrease the number of car particles the scene will also speed up.  Keep tweaking these values until you find a balance that works for your machine.  

Your computer has run out of memory trying to render the scene.  Try decreasing the number of particles in the scene or turning off sections until the view corrects.  

All you need to do is relink the hdr texture used in the world shader.  Simply open the world shader node graph and find the texture node that is accessing the hdr file called kloetzle_blei_4k.hdr  Click the file folder icon next to the name and navigate to the file in your download pack.  Now the purple skies will go away.  :-)