The insurance part depends on how you are going to be operating. The cost of it isn't impacted by whether or not this institute or that organization says you know how to detail. The cost is based on the risk. If you are operating out of your house then it may be possible to include it in your homeowner's policy. If you are mobile you may want to get a seperate policy. The same holds true for a brick building business. The best suggestion I could make for you is to call whichever company you currently have insurance through and see what they suggest. You can also check online and get some quotes from different companies. Insurance rates are going to vary by state and by business structure, so it is hard to get an idea of what it will cost you without more details about your plan.
My thoughts on certification is that it is nice to have if you don't want to take all the time it costs to figure things out the hard way, but it isn't necessary for obtaining jobs. I haven't had one single customer who would know the value of any type of certification that I might have. They judge me by my work and not on how I learned to provide the results. It may be that in order to work for certian dealerships or shops that they will require you to have a certification of some sort, but it isn't required to do business on your own. I'm sure it might lend some credibility to your business, but if you are carrying insurance, are registered with the state and carry an active business license for your trade, then additional certifications just help show people that you are serious about the industry.