Payment Provider Agregator Backend Web Development

Hello. We are looking for a senior back-end developer to create a progressive web-application for an aggregator of payment providers service. We are open to any stack that will be work the best for our task and already have a detailed tech requirement for the system. This is a long-term ongoing project, so some changes may be applied and they will be paid. And because of that it would be great if you can also suggest some better solutions while you will be working on project too.

You will need to create a prototype and then a whole system for a web-application including DataBase, Logics and Views for all the screens. Ready layouts for screens will be presented later (at the moment we already have designs and layouts development is in-progress)

What do we need from a developer:

1) Five or more years of proven experience in back-end development
2) A portfolio with actual projects that are able to be checked in action
3) Deep understanding of how to create a complex, scalable and fast system from a scratch
4) Similar works to this one on which you've worked before
5) Understanding of gits and using a git-repository for version control of your layouts

What do we have:

1) A detailed tech requirements and specifications for the whole system
2) A project team which will be happy to answer on all your questions
3) Ready designs for the whole project
4) A server for work and deployments

What do we expect to have as a result:

A full access to related git-repository, a deployed application on our work-server and ZIP-archive with source files of your ready-to-work application. The code should be clean and readable with comments and it should be written according to the BEMmethodology. No errors should be presented in your code (will be checked by validator).