If someone has to give you money to get some kind of access to the game then that would count as a purchase. That counts even if it’s early access and the final game will be free. Essentially–if someone at any point has to pay to access the game in some form then that is a purchase. If the game is always free then there’s no royalty on the donation.
I’m not sure what you mean by foreign assets, if you’re thinking of just random stuff you’ve downloaded online, then that would not be legal to use without permission even if your game is free.
If your game is required to be open source then you would need to make your UE4 project files available, though remember you can’t redistribute UE4 itself, so just your project files.