Hello everybody, I'm quite new to drupal and Commerce and I'm french so sorry in advance for my english.

I'm currently building an e-commerce store for a person that want to sell unique objects (like hats, accessories, etc...).
I'm using commerce kickstart 2 and I would like to do the following :

Since there will be only unique objects on the site, I would like to show a "order custom product similar to this one" button next to the "add to cart" button on product pages. And when a product has been sold (so when the stock level is 0) I would like to only show the "order custom product similar to this one" button on the product page.

This "order custom product similar to this one" button's purpose is to give the ability to the customer to ask the seller (my employer who make unique objects) if it is possible for him to make a similar product. So this button would link to a form with a "special requirements for the custom product" field. And when the form is filled by the customer, an e-mail is sent to the seller telling him :

" [this customer] would like you to make him a custom product similar to [this one]

Click [here] to validate his order or click [there] to answer to your customer "

In short, I would like to create an exchange between the seller and his customers.
How can I achieve that ? I hope I've been clear enough... Thanks in advance for your answers :-)