Thank you for the encouraging comments. They are really helping me staying motivated.
I decided to declare the fourth step completed even though there are still some issues I am not happy with.
The damage system is working (The player ship takes damage but does not get destroyed in the video). Every object has 8 hit zones: Front-Top-Left, Back-Bottom-Right etc. This should give the player a lot of possibilities to position his ship cleverly to absorb as much damage as possible before getting destroyed. However, this system has one big disadvantage: If the borderline of two or more hit zones is pointed towards the enemy, it is almost pure luck which zone is hit. This makes ships a lot more durable than intended. I will correct this in the future by distributing the damage across the hit zone itself and its neighbor hit zones depending how close to the border the hit occurred.
It is now also possible to save a ships current state and create ships with modified attributes at runtime. Strangely, ships created this way ignore all input to the movement component. I tried to find the cause of this but was not successful yet. Ship instances placed in the editor do not have this problem. Maybe someone has an idea and can point me in the right direction.
I also implemented a primitive start menu but decided to postpone all further HUD development until UMG is available.
Plans:
In the next weeks I will mostly do some cleaning up. I discovered a lot of code needs improvement and I think I should take care of that as soon as possible. So that will be the top priority for now. The second goal will be to create some kind of demo mission that I can use in the future to demonstrate all features. As a bonus I will try to make a few generic assets to improve the visual variety. I hope I will learn enough about modelling and texturing this way to make decently textured space ships in the future.