Ask HN: Testing new market with landing pages, what % conversion to look for?

We've been developing a product for the last few months which has a large but saturated market. We've decided that with a small twist, we can be in a completely blue sky environment. We've shown our designs to a few potential customers, and they love the new idea (though they also loved the old idea, just didn't know all he other options that existed, so I'm taking their feedback with a grain of salt). I want to test the market and get a feel for how large the market may be and how much customers may be willing to spend. I'll be creating a few different landing pages, focusing on a few different customer types, as well as buying adwords to direct traffic to these pages. However, I'm not really sure I know what I'm looking for. What would be a good click-through rate? As we'll be asking for a potential customers email address, what percent of visitors should we expect will give us their e-mail address? Is there anything else you'd recommend we do? Am I going about this all wrong? I've been searching online, but everything just says "setup a landing page and drive traffic via adwords", but once you've done that, how do you know if you were successful?