I'd wager most still successful businesses weren't started with much of a plan, especially if they offer services instead of products. I started mine by paying less than a hundred bucks to incorporate on a whim, and was successful for many years. The only reason I shuttered it is because it led me to programming and I went off in that direction instead. There are many such examples everywhere you look, so it's not just anecdotal. I'd say that all one needs is determination and a vision.