Any less experience and potential clients may not take you seriously. In addition, I think your resume should include some project management experience. The most valuable experience would be jobs where some technicians, designers and perhaps assembly workers report directly to you. Good verbal and written communications skill will also be needed. As a consultant you will be required to conduct meetings, manage people and write countless engineering and progress reports.
Many projects involve more clerical work than actual technical services.
One job that would be a great place to learn about the consulting business, without the risks of being self employed, is a contract service company that provides consulting-type services. These companies may respond to proposals on complex projects that require many engineers and technicians. Often these companies perform many of the same services that individual consultants might offer. Working for one of these companies would be a great launching pad for your own business. In this day and age you must be skilled at using a computer.