Hi,
Since we made crypto currency before so,
1) We recommend to make it from start but if you do not have budget then github code should do the needful. But your budget is to be increased if you are looking to make bitfinex.
2) It should follow blockchain
3) We would like to make decentralized trading engine. We plan to make web platform.
4) We are planning to make peer-to-peer (P2P) bitcoin exchange platform.
Lets discuss as space is too less.
Regards,
Mit-