Create/Code Additional OpenCart Payment Gateway

Create/code an OpenCart Payment Gateway Extension for a client I've set-up with OpenCart 1.5.6.1 for their shop. The gateway of preference is local one to them and they do not offer an OpenCart solution as to me looking for someone that could integrate their SOPA based payment gateway solution to the shops front- and back-end. Previous experience with OpenCart and Payment Gateways a must as to create a fully integrated and working solution. I've attached the suppliers Gateway Raw Connection documentations as reference, they also sent a PHP version of their solution that will be shared in due time. Considering this a simple project for someone with the right experience, potentially adapting one of the existing similar solution from OpenCart or he/she worked on in the past, I'd be looking at a budget around 100-250£, In case of questions let me know.