Hi,
You'll probably first want to have a custom field of the table "product" and of the type "simple dropdown" with two values: (exc. VAT) and (VAT Free).
Then, in each product, you can select the value you want in that custom field.
You'll then have to edit the file "listing_price" of the view "product" via the menu Display>Views.
That's the view file which display the prices of the products on the listings and the product pages.
There, at the end, you can use such code:
<?php echo $this->row->XXX; ?>
where XXX is the column name fo your custom product field.
That way, it will add the text of the custom field at the end of the price.