The only change to how Build works is that a land without Invaders might Build (if it meets the listed requirements), which normally is not the case. Tags: Build, Kingdom of England
The order does sometimes matter for Ravages, due to Blight cascades, flipping the Blight Card, or uncommon effects that change the amount of Invaders in a land that is already Ravaging, but the order won't change which lands Ravage even against Kingdom of Sweden Level 5+ or Tsardom of Russia Level 3+.
If an Event tells you to perform a single Build action - eg, "On Each Board: Build in a land with Dahan" - you don't check whether that land has Invaders.
But if it tells you to resolve an entire Build Step / Build Card for a particular terrain, it works just like it would if you had a physical card - you're making a list of which lands match, so the normal Build/Explore restrictions apply. The Events "Fortification" and "New Cash Crops Take Hold" work this way (and do not usually build in empty lands), though it's not how they're phrased. (See the Errata in the JE rulebook p. 27) Tags: Build, Explore, Ravage, Events, 2 more...
If there is more than one Invader Card in the Build spot, then Towns and Cities added from the first card do count for the second card. Likewise if there is a build from England's High Immigration: Any buildings added from that rule will count for (e.g.) the regular Build. In general, you check what lands will Build at the beginning of looking at the Build Card.
The step is still the Ravage Step, so anything that specifically happens before, during, or after the Ravage Step will still happen at its normal time (but no Build Step effects will apply). Nevertheless, the resolved Invader card will obey all Build Card rules (both normal and Adversary-driven), cause Build Actions, etc.