Personally I’d use an Actor Component class and add it to the Player State.
Later in your development cycle you’re going to start working on game disconnect scenarios. If your inventory is stored in a level BP, or Character it’ll be instantly wiped without a chance for retrieval on reconnect. Best place is Player state which offers an opportunity on the server side (C++) to do a backup before it wipes its copy of you.
Anyways in the player state class you simply add it in the components tab like you would any other component. Then use the generated reference (variables section) to manage it.