Hi,
I can't help you in customization even though you pay for it as i am extremely busy with my current projects. I will give you lots of delay which i believe you don't like it.
And our free support doesn't include any kind of custom changes. If things get worked exactly how they work in our DEMO, our free support ends there. But the PHP files you need to modify (for checkout page layout) are this:
ROOT/templates/<yourjoomlatemplate>/html/com_virturmart/cart/default_pricelist.php
ROOT/templates/<yourjoomlatemplate>/html/com_virturmart/cart/default.php
and
ROOT/templates/<yourjoomlatemplate>/html/com_virturmart/user/login.php
(for the login form and the "checkout as registered user", "checkout as guest" buttons..)
I will definitely recommend you to use "firebug" plugin in FireFox Mozailla for CSS and graphic customizations, it will save you tones of hours.
Refer to this page about where to get that plugin and how to use it:
www.flexiblewebdesign.com/forum-virtuemart-2-template-puremart-support/342-using-firebug-for-customization-or-troubleshooting
Regards.
Forum Support.