I’m always wary of basing campaigns around movies because there’s always the potential that the players will make radically different choices. Like, if the players decide not to stick around for the next fog, will that be okay?
To answer your question, campaign level is a function of the types of monsters you’re looking to include. Since these seem to be mostly humanoid soldier types, you probably want to stay low enough level for that kind of combat to be a danger.
Starting at level 2 or 3 would probably be appropriate to maintain the challenge. On the other hand, if you’re planning to pit the players against an army of dozens of soldiers, you may want to start at level 5 so they can mow them down properly.