Hello, I've been a Joomla developer for 12 years. It should be pretty straightforward to model and existing module and adapt it for use with PayNow's API.
Not only have I worked with Joomla for over a decade, but in that time I've worked with several payment integrations, and several times that number of miscellaneous API.
I'd love to chat a few minutes and get some details.
Regards,
Joseph Cardwell