Please do not post the whole codes here...I think we have it, unless you have modified codes ;)
1) on custom_checkout.php you need to put in the variable for showing total price, I add this line myself and seems working. (I am using the CRE, modified oscommerce)
<?php echo "<h1>£" . number_format($products_total * 1.175, 2) . " (inc VAT)</h1>"; ?>
2) option to build the pc? not understand what is that....
3) for removing or modifying the more info button, you need to deal with the java.php file...search the more info words in that file and you will see it ;)