When you say you are considering.... what does that mean? Have you done any research? Do you have any experience running a labor intensive business like that? Any experience washing cars?
Why hand wash? Why that location?
To survive, you will need to be able to recruit, hire, train, manage and lead a large team (30-75 people).
Hand washing is a niche service. Does well in some areas but not universal. The amount of time it takes can make it hard to get customers to come frequently.
From my experience in full service car washing, a hand wash is a daunting challenge. I wouldn't want to do it. Companies are moving towards more automation, not less. I'm not convinced that is a trend that you want to fight.