Before anything else, I would heavily research the potential market. In other words, will such a business even be viable? I used to build custom machines on the side and that has all dried up. Everybody wants lap tops these days. Even in an area as large as Dallas/Ft. Worth TX, finding a small computer maker is a chore. We do have some smaller chains such as Microcenter and Fry's that sell components for enthusiasts. They also build their own brand machines to order, but it is a very small percentage.
You could also go to work for someone else for a year or so and learn the business that way.