When I place a lot of high poly actors that are set to “hidden ingame” in a level. Are they all loaded once the level is opened or just when “hidden ingame” is deactivated for that specific one ?
What is “a lot”?
The engine can handle quite a few actors when they are not displayed. They will however all be spawned (which might take a lot of time) and when they have their tick enabled they also might reduce your framerate even when not visible.
If possible, try to use components instead of actors, they are definitely cheaper.
I´m planning some sort of configurator where I need to be able to change parts. So basically only one of these actors would be visible at a time and each actor is unique. Because one actor can contain various objects I can´t use the “set mesh” command. Well it could work that way but that would mean a lot more work in advance. So my next idea was to use actors I hide or unhide on event. For the moment we´re talking about maybe 20-30 actors. But I want to keep it scalable so in case I need more in the future this method is still acceptable. But if I understand it right, once I set the actors ticks to false it´s shouldn´t impact fps performance when they are present in scene and hidden but the loading time of the scene gets longer with every actor ?
I don´t think in my case creating one actor that holds all the geametry as components would make a big difference as it´s individual geometry that can´t be instanced ?
With these numbers you will not notice any impact. 
When you try to spawn more than a few hundred actors you might start to notice performance problems, but this is absolutely fine (even with ticks enabled).