Reading through your list of points, it sounds like you're trying to justify not just trying and seeing what's possible. There's no golden rule or answer to starting a business, but there are plenty of successful businesses that started as a small part-time endeavours.
As a start, 37signals did it with only 10 hours a week:
http://www.37signals.com/svn/posts/1078-it-doesnt-have-to-be...