Hi,
This is Shakthi TG from Chennai India, and am a medium level expert in Magento and m2e. I have developed a few modlues for Magento and can build the payment gateway as you had specified in the PDF
I also read the 381808 Merchant operations that outlines the guide to integrate the payment gateway to Magento and their various call backs and API's
To be specific, you had mentioned that you have a few custom modules including custom payment gateways, so just wanted to know if you want to add this gateway along with other payment gateways that exists in the system or you want to remove them and have only one payment gateway.
I am an independent developer having about 12 years of expertise(ience) and am confident to handle your project to the best of my knowledge possible,
Looking forward to work on your projects,
Best regards,
TGS