A car wash can be a very good business. Some things I would look into:
1. Does owner have legit books so you can see what kind of revenue the car wash actually does, or is he pocketing most of the money and not reporting it?
2. What kind of shape is the mechanical equipment in? At 25 years it may be about shot, and this may be his reason for wanting to sell now.
3. There are people that sell car wash equipment. They would likely evaluate the condition of the current facility and let you know what updates are needed, what approx. expenses for new equipment would be, etc.
4. You need to keep legit books and make your money post taxes if you ever expect to be able to easily sell the place down the road.
5. A car wash owner needs to be kind of a jack of all trades handyman as you will have constant little plumbing, electrical and housekeeping issues to deal with.
6. Know your cost of utilities and how that all relates, taxes too.
7. The constant "wet" conditions means things wear out quicker, require more periodic upkeep, painting, cleaning, etc.
8. Exterior appearance needs to look sharp to attract business; clean, well lit, updated signs & paint, etc.
9. Be prepared for the guys that come in and blast off their muddy 4wd trucks leaving you with plugged drains and a stall full of mud.
10. I'd plan on someone working there at least 4 hours per day every day to keep the place up, and pay yourself or that individual a fair wage for this time.
11. Is there mowing and snow removal associated with the property?
None of this is meant to discourage, just to make you aware. I know several people that have done very well owning car washes.