What kind of account do I need to open with Paypal and Authorize.net to perform some tests on my side for my proof of concept?

In order to test Online Payment/Registration Pro module without actually submitting real CC data, you can create a PayPal Sandbox account. This requires that you go to an entirely separate PayPal site and create a new and separate account from your main PayPal business or WPS account. Here's the site: http://developer.paypal.com.

For Authorize.net you can apply for a test account here http://developer.authorize.net/testac.... When you have your authorize.net test account enter it under gateway section in the module settings and check the test mode checkbox.