@Stephen Brown
For starters, find local chemical distributors and call a few of them. They know the market and might have insight on volume the wash had done and the condition of the building and equipment.
I’d look into traffic count and speed of road. Is there a stoplight that keeps eyes on your location a bit longer or so people fly by at 55mph?
What is the population within 3 or 5 miles? Thats where your customer base is.
A good chemical rep should be able to offer you insight on traffic and population.
This data will give you a sense of volume.
A working wash that’s operating regularly should be valued loosely at 3-5x yearly gross sales. In this case, I’d anticipate discounting with that as a sterling point.
Also consider what the costs will be to get it up and running.
On the back side, know you are building a business not a passive investment. It may not be a full time job, but someone will need to be there at minimum most days of the week.
Good luck!