The template that displays the product details is skin1/customer/main/product.tpl
If you amend the code by adding by a conditional around the add to cart/price part along the lines of:
{if $smarty.section.products.index eq 16}
email me for details
{else}
the normal price display bits
{/if}
where 16 is the product id that you want to show the email detail for.
If this is more than one product then change it to:
{if ($smarty.section.products.index eq 16) or ($smarty.section.products.index eq 19)}