I was in this exact same situation two years ago, only with an eCommerce platform which shall remain anonymous. The client had gone through 4 companies, trying to get the project built and finished. Nobody had been able to wrangle it clean.

Reluctantly, I took on the project, and started working through it. What I initially estimated would take me a month to untangle ended up taking a year. That's an entire year in the snake pit. And since it was eCommerce, there was serious money on the line when it came to bugs. And there were hundreds that I found.

Just understand the commitment you're making. Make sure your client has the money and the time to make things right. Ultimately, in my project, the client insisted on quick hacks to keep competition at bay, and the code dissolved into a mess. I decided I couldn't keep working with a codebase that was never given a chance.

Understand what you're getting yourself into. Because you're taking over the responsibility of loading a massive crap ton of software into your head for diagnosis. How long do you want to have fragile, crappy, lazy code in your head? Forget about bring superman, you're not going to save a million lines of code, you're going to become the builder of the hacks that work around absurdity. Make sure you understand that.

The burden of broken code you're responsible for, that's always broken in production is like nothing I've ever encountered. Make sure it's worth it.