I have a problem with the display of inventory sheet. It shows only the number above 0 - I wish it also showed "Only 0 left".
Not sure what do you mean by "inventory sheet". Can you describe more precisely what do you want to achieve?
I would like to Magento showed the number of products even if the magazine is empty: Number of available products: 0 Units
That message is not standard in Magento, so you probably have a custom theme. Because of that, I cannot help you by saying file names where you can change this behavior, but I can give you advice just to search all files in app/design/frontend for a specific word, for example, "Only". That's important because on that way you'll be close to the code which deals with that message. There you'll probably have if statement which inspects wheater qty is above zero or not. There you can change your messages.