Without being too silly, my best suggestion is your brain. Second, study the niche you are wanting to enter and talk about it with your friends over a beer. You'd be surprised at what kind of info you can get from the "non-techies". That said, perfect is really hard to achieve. I settle for good 'nuff and get 'er done and get on with business.
Depends. Most are owned, but not gone. I've acquired a few gems over the years from folks who had no use for them. Not too expensive in the grand scheme of things.
Just because a domain is registered, does not mean you can't acquire it. Do some research. If it is not being used, and has been dormant for years, try to buy it off the owner for a reasonable, yet affordable price.