Depends on your objective. If you really just want to know whether the idea is viable, you'll get the most bang for your buck by keeping things email-capture-only or putting up a "smoke test" where you have a Buy Now button that redirects to a "it's not available yet; but leave your email if you want to be notified when it's ready"... and see if you can get anyone to click "buy now" by clearly articulating on your landing page what the product will do for them. This also lets you test pricing sensitivity. If you can't get anyone to leave an email or click "buy now" based on a really good description with images (or even better a video "demo") then building the actual product is unlikely to be worth it.