Best way to have object with many parts drop in ready?

Getting a single object drop in ready is easy. Change the origin to the bottom and done. Let’s say I make a little cart with some wheels. I want to be able to select the cart with the wheels in one click and have the origin at the bottom of the cart so I can quickly drop it into any scene or if I drop it in a scene it is all one thing. How would be the best way to do this fast?

Below is what I’ve come up with so far. I got the objects all parented to the outside box, changed the origin of the box so I can snap it to the plane easily, and turned the box to a wireframe. What I don’t like about it is it is a lot of work if it is done with hundreds of objects. It seems there should be a make bounding box parent of selected objects command or way to script it. Second I would like to be able to select the box like if it had faces still and was not only a wireframe.
grouped drop in example1.blend (661.0 KB)