Automatically set featured image

I need a plugin which automatically takes the first image of a WP post and assigns it as the "featured image" of the post. This needs to be done whenever the post is updated. Furthermore, the plugin should update a bunch of previously posted blogposts, with a button. When the button is clicked, the plugin goes through all previously posted posts, checks for the first image, and assigns it as the "featured image" of the post. I can pay maximum 50 USD for this job to be done.