Kobra is right in the sense that you can't strip away unwanted functions, because of the sheer inter-relational complexity of eComm software. Everything should be left intact, and rather, you should be looking for "leap-frog" techniques than trying to get rid of functions that can - safely - just be left dormant.
If your products have no attributes to be selected (such as size, or colour), then embedding the following url into an img src link will add what ever product straight to the cart.
PHP Code:
index.php?main_page=index&action=buy_now&products_id=XXX
SO... your home page can be hard-coded (in define_main_page.php) to be something like:
HTML Code:
<p>YOUR TEXT WOULD GO HERE</p>
<a href="index.php?main_page=index&action=buy_now&products_id=123"><img src="images/pay_by_card.jpg" alt="Click here to pay by card now"></a>