If all you want to do is diversify your portfolio....there are WAY easier and more profitable options out there then building a carwash. I would encourage you to explore other options. I heard a story one time that a room full of wal mart corporate big wigs were being pitched a carwash idea, it was a lengthy presentation, with the start up costs, equipment, soft costs, etc.. all broken down, avg ticket price, expenses, the works. And finally the bottom line number was circled and pointed out to the room, there was a moment of silence as everyone was taking it in, no one wanted to be the first to say yay or nay. And then one of the big wigs stood up and said "Or, we could just find a way to increase the profit margin on our t shirt sales by 5 cents and make double that." And that was the end of that.
My point is that building a carwash just to diversify your already nice portfolio is more often than not a recipe for disaster, and that there are many easier, and less risky ways to achieve that.