I agree, the website is very confusing. I brought SC-WWII Global Conflict & the Assault on Democracy & Assault on Communism Bundle which has all the improvements of the game.
The 1939 world at war scenario is both the European war & the pacific war. The Pacific part of the scenario is the same as you get in Strategic Command Pacific Theater but better if I remember rightly.
You just need to buy SC-WWII Global Conflict & Assault on Democracy, if you don't want to play a European war, but that's up to you.