These four things are mandatory for starting a bitcoin exchange trading website.
Bitcoin exchange software or source code
I don’t know in detail about the other three points shared above. But I am expertise in the second one. (i.e technical development)
As many told, there are open sources available but it is too risky to operate and can be tedious if you get into that. A lot of technical complexes are there if you go with open source. On the other hand, you can pick the software which will give source code access to you. I am a blockchain developer working at Cryptoexchange4u, offer cryptocurrency exchange software with a lifetime license. To start from scratch, you can contact the official website of cryptoexchange4u.
Hope you will find it useful.