About legal assistance, you should know that if you plan to use fiats you won't be able to be anonymous, KYC is mandatory regarding the usage of FIATS and Bitcoin (only in some countries).
If you do it regardless of what i said you will never be legal, and you could even be prosecuted for money laundering or terrorism financing.
This would really be the case if theres an anonymous decentralized exchanges since we are still in involving on fiat when we are building an exchange site so this means we should follow the rules regarding on fiat terms which you will really need it because you cant execute normally if funds/fiat are lacking. You will be facing such problems if you tend to build one.