Hi,
Since you have server already built so, to make the communication using JSON data, I must have the web services to be called. Am I right? If yes, then who is going to provide me those APIs?
Moreover trading of currencies involves CALL/ PUT features, stop loss, mitigate Risks, having CRM for customers, graphical output for performaces etc. So, this can no be an easy project to be completed with in your budget.
Lets discuss the scope and then make a final decision.
Regards,
Mit-