I'd test the waters and see how much business I was doing before I did anything. I can't tell you how many times there is a post "I'm thinking of starting a part time detailing business..."
Its a hard business to break into and get going. Everyone thinks that they have that special thing that they will do/offer and have the most successful business.
I'd try it for a month. See what kind of numbers/money your pulling in. Then see if its worth it. See what your cost/profit ratio is. Get quotes on insurance to see how much it would cost you every month. Insurance is expensive and you could basically be breaking even if your making a monthly payment to them and only doing 3-4 details a month.
If you get a DBA or a Tax ID #, your on the IRS radar. Now, I'm not saying to not pay your taxes, but IIWM, I would wanna see if I could make a go of it before I have to start filing quarterlies, and recording all of my income. There are a lot of people that start out without the DBA/Insurance/ID #'s and do fine. Once your business grows and you think you can make it (which is damn hard once you take into account ALL your expenses) then looking into going on the books.