Hi,
I have good experience building WordPress plugins. I am confident I will be able to develop the payment gateway. Previous plugins I have created which are relevant to this project had integration with several API, secure communication, encrypted information exchange, integration with several payment gateways, like PayPal, stripe etc.
The work as you already know will be research-based. Being able to communicate a payment gateway is quite crucial. If you do not have such resource, then we might also look into creating an API ourselves (not sure if this is an investment you are prepared to take)