Back in my research days I was going to apply for a certain patent, and spoke to a lawyer at the university about monetizing it. He ended up telling me that it can actually be dangerous to be too exhaustive in your research. You want to do a general search of a few databases of your general idea, but if you're very concerned about the idea you'll need to sanitize it by have someone else look and inform you if they find anything that's specifically applies to you.

If you find something that might apply to you, and it could be shown that you knew of the patent before you started (e.g. you told potential investors "There are only a few patents that could hold us up, like...[blah]...but we're confident it doesn't apply to us" and continued to try to monetize it, you could be up for punitive damages. Painful indeed.

Have a knowledgeable friend who you trust and would look out for you do the searching. They'll probably put in just the right amount of time - quite a bit, because they care about you, but not so much that it would impact them too much - and you're free to claim ignorance if anything comes up later.

If anyone knows anything wrong with this advice, it would be great to knock some sense into me at this point!