If I were to purchase an extended license for a stock photo, is there something in writing that I get stating I have done so? Just in case i'm asked to show proof?
It will be listed in your license history on the Stock Website showing you have an an extended license.
Any license you will buy will be reflected in your account indifferent of the license you buy, as you could be asked to produce proof of it. As a buyer, you have a contract with Adobe who has a contract with the producer. And you have as a buyer all proof you need in your account.