This module here seems like a good option for a single page checkout -> http://www.opencart.com/index.php?route=extension/extension/info&extension_id=5754&filter_search=one%20page%20checkout&filter_license=0 It is free to download (opposite to paid modules) and has a simple documentation about installation. You just have to unzip the file you download and swap the corresponding files on your server.
This module seems to be a good one.Have good list of options. baltokoski.com.br/products/Opencart/OpencartOnePageCheckoutPlusCustomRegistration