Hi, exchange code is used to make money. The only fully functional open source code i know is peatio and it's really hard to modify. It took me almost a month to get it to support multiple coins let alone customize it to some of my specific needs, eventually I gave up and started from scratch. I have been developing my own exchange as part of a much larger project :-
Xequium. It has an exchange built completely by me, and I think that you should follow a similar path. This is mainly due to the issues mentioned in previous responses but also because you will learn a lot in the process and be able to quickly diagnose a problem should one appear. By designing your own database schema, security model and features you have the benefit of knowing your system in and out, then if you do it well you are more likely to attract users.
Note:- developing/running a proper exchange efficiently is a taxing job, it requires a lot of time,patience and coffee. I had the advantage of a background in CS and SE sand massive experience coding for crypto projects , but diligence should cover that for you.
Some things you should research :-
1) securing a server
2) securing a wallet
3) communicating with a wallet
4) storing data properly in a database
5) differences between what will be done server side vs client side.
Checkout the exchange part of my platform :-
It goes live as soon as i finalize work in other areas of the project, raise some funding etc.