Is there any way to use soft delete on posts/threads in IPB? Do you have to install a addon? (if there is any).

You can enable the trashcan in the ACP so deleted messages are sent there, or you can set topics/posts invisible.

Is it possible to allow a only a certain admin/moderator level to delete posts/topics completely, while the rest of the moderator levels can only send to trashcan/set invisible?

Yes.

Ok, thanks