I can tell you a few things:
- a simple exchange it is easy to get, but it relies on 3rd parties wallets
- if you aim toward a real trading platform, the things are more complex (and expensive)
- if you want to generate and handle wallets, the things are extremely difficult

I worked about one year along with other developers to build our own exchange. We picked the best core we could find and moved on. Initially I though it would take about 1 month (max) to have it ready, it finally took me an year.

What is important to notice, as I though a lot about this before starting the project: we do not store private keys. We rely on the bitgo api to secure the coins and to have a backup record of each transaction. So if you want to store wallets, that it's a huge challenge.

I have worked for more than 10 years as a developer so I understood pretty well the security requirements from the very start. If you are not a developer yourself, you will need to trust other people for dealing with sensitive code parts, which potentially can leave the door open to hackers. So be extremely aware.

IMO, at this point you could buy a PHP based exchange script, which relies on 3rd parties wallets. The disadvantages of such an approach are obvious though: you depend on a centralized exchange rate and pay fees to the wallet providers. But even in this scenario you need a lot of security, as you will still store some personal data that could potentially could leak funds in case of hacking.