You don't need to do anything to start a business as a sole trader. *edit* tell the tax man* Just trade and declare your income. If for some crazy reason you want to start off as a private limited company, go and see an accountant. They'll sort you out.
There will always be a market for car parts because most people will always need and want a car and they will always need parts replacing. I can see more of a market rising for this type of business in the near future as the credit crunch will only help the sale of cheaper and older cars that are heading towards needing new parts ect.