Possible Duplicate:
When is a BIG Rewrite the answer?
Software rewriting alternatives
Are there any actual case studies on rewrites of software success/failure rates?
When should you rewrite?
We're not a software company. Is a complete re-write still a bad idea?
Have you ever been involved in a BIG Rewrite?

This is an area of discussion I have long been curious about, but overall, I generally lack the experience to give myself an answer that I would fully trust.

We've all been there, a new client shows up with a half-complete project they are looking to finish and launch. For whatever reason, they fired their previous developer, and it's now up to you to save the day.

I am just finishing up a code review for a new client, and in my estimation is would be better to scrap what the previous developers built since and start from scratch. There's a ton of reasons why I am leaning toward this way, but it still makes me nervous since the client isn't going to want to hear "those last guys built you a big turd, and I can either polish it, or throw it in the trash".

What are your general rules for accepting these projects?

How do you determine whether it will be better to start from scratch or continue with the existing code base?

What other extra steps might you take to help control client expectations, since the previous developer may have inflated those expectations beyond a reasonable level?

Any other general advice?