The custom thankyou page in html is easy:
to the user defined variables at the top of checkout.php add the filename of your thankyou page:
$header = "header.html";
$footer = "footer.html";
$returnpage = "/";
$thankspage = "thanks.htm";
$youremail = "email@example.com";
$csvfilename = "orders.csv";
$csvquote = "\"\"";
$mode = "BOTH";
at the bottom of the page where it now generates the thankyou message, replace the two includes and the echos with
//# Send email conformation to the customer.....
$mailheaders = "From: $youremail\r\n";
$mailheaders .="X-Mailer: PHP Mail generated by:NOP Design Shopping Cart\r\n";
$subject = "Order Confirmation";
mail($b_email, $subject, $strMessageBody, $mailheaders);
Now to generate custom messages for errors, you will have to edit/customise the header & footer files used in the include() for each error page. There are 2: doerror and doformerror. Of course you can used different headers and/or footers for each message, just remembering the echo statements will have to appear in the html page somewhere.
Have fun...you can't break it permanently!