It's easy to get the tax assessed value. As far as income verification, you do that by looking at 3 years income tax returns for the business.
Car washes typically sell for 3-5 x gross revenue. But that's a guideline and you want to consider many more factors in determining the feasibility of the purchase. Age and condition of property / equipment, car counts, is it well kept and fully functioning, how are the Google reviews, etc.
Go there and sit in your car on weekdays and weekends, day and evening hours. Observe the place. Is it busy? What are the customers like? What is your own experience when you wash your truck there? Is the pressure good, water hot,
soap rich, foam brush like shaving cream, good scents, vacuum suck well, etc?????