Hi there,
To make it display a button that says "order now" instead of "buy now", create a different button and upload it to your ZC image folder. Make sure the file name is the same as the current button image file. This will replace all "buy now" buttons with "order now" buttons.
To display a "buy now" button on available products and an "order now" button on upcoming products is not possible without extremely significant hacking of some core files. It's just semantics anyway - in both cases, they are ordering and paying immediately. The only difference is when the product will ship - which is made clear by the product available date.
The product available date will show up automatically at the bottom of the product description, just below the buttons for "review this product" if you have that enabled.
The message showing the product available date will automatically go away once this date has passed. You won't need to manually update it once the product becomes available.
I realize you don't want to have to go back and edit all your products that this would apply to, but that's the only way to make this show in the product listing as an automated function. Even if you hacked the code to display this elsewhere in the product listing (like in another column as you said you'd like to), you'd still have to go back and enter the date available.
If you're concerned about the line of text with the product available date being too easy to miss (which is why I assume you want to place it in it's own column), you could always edit the language file to make the text stand out in a bright color and bold font.
On the other hand, if you still want to place it in a column in the listing, instead of leaving the text where it is, you'll need to add another column to the table in the file where this information is displayed.
You'll need to have an understanding of html tables and html and php syntax in order to do that. You can learn a great deal about this by following the code that is already present in the file.