Default Value "Free" when no price is defined.

It's in the configuration under the tab languages that you will be able to edit the translations in HikaShop:
www.hikashop.com/download/languages.html#modify

you can use HTML in the translation strings. Just make sure that you use simple quotes: MY_STRING="<a href='my_link'>my link</a>"

you can add %s to the string in order to be replaced by a parameter.
Then, in the view where the string is used, you can add a parameter to it.
For the FREE_PRICE string, you can edit the file listing_price of the view product via the menu Display->Views.
There you can replace the line:
echo JText::_('FREE_PRICE');
by:
echo JText::sprintf('FREE_PRICE',$this->row->product_id);
and then, the %s in the translation string will be replaced by the product id.