Hi
I'm a French developer with very good knowledge of PHP, MySQL, OScommerce, Prestashop...
Regarding login to admin section I would suggest to use htaccess/htpassword as it is much more secure (against brute force). And we can have many users if needed (not just one).
Paypal Api problem will be fixed as well, best solution to solve the problem is to look at the Paypal log and fix it.
Job can be done within 2 days.
Best regards
Alex