If I understood correctly, you could do the following:
Let’s say you have a widget where the player can choose a weapon for his unit. You could either use a map (not recommended if game scales up) or use a simple data table with the structure “Display Name of the Weapon; Corresponding BP Class of the Weapon”. Use the DT in the widget and if the player makes a selection, you look up the corresponding BP Class and spawn it in the world (at location 0-0-0) while setting its owner to the player.
This will trigger event “Begin Play” in the Weapon Base, here you can cast/interface to the player (as it is the owner) and there you can run “Attach Actor to Component”.
I hope this is answering your question 