Hi,
From your one line of description, I understand that you want to make a platform like bitpay. That is itself a payment gateway + providing REST APIs for integrations with merchants, ecommerce portals and registering user for bitcoin /altcoin exchange and transaction.
To make such huge system, we need to make exchange platform where the market rates will be coming from APIs.
Here, Block chain algorithm is to be used for peer-to-peer transaction. We have done development of crypto coin, so we are 100% sure about it. 128-bit SSL security is recommended.
Lets discuss if you can increase your budget.
Regards,
Mit-