NEW YORK — Columbus is the sixth best metropolitan area for starting a career, according to a report by Bankrate.com.   Bankrate studied the biggest 50 metro areas and evaluated them based on several factors young people should consider when starting their career, including the job market, potential for long-term career building, affordability and lifestyle.   Columbus' best scores were for job market and affordability.   The top five in the report are Houston; Minneapolis-St. Paul; Washington, D.C.; Milwaukee; and Dallas-Fort Worth.