Very smart to talk with the equipment/chemical rep! I would for sure ask to turn on the systems, and plan to make a day of it. I would ask the rep if he is willing to be onsite for that and help get it running. If it can. It is not just turning the water on. You will want to test soaps to make sure there aren't underlying problems. There's a lot to getting soap out of wand in a way that looks good and cleans cars.
If you can turn this on as a car wash (also, is there even still demand, or did someone open a new self serve down the road?) and get some income coming in, it will allow you time to decide what to do with the lot and building. Though even if it worked, the offer should really only change in terms of the value of the equipment (which could be a few thousand). With no business, theres not many actuals there. Its also likely that if it shut down, there was a reason and it may have a bad reputation. Either way, after 6 years, people have forgotten about it.
One of the challenges, is that car washes are more or less a single use property. Its tricky to convert the structure to something else. If the location is good, that could be worth something. Its also possible if there is demand, you could start the business back up.
Talk to the rep about converting 1 or 2 bays to automatic washes. That will allow for more revenue, though there is certainly a 6 figure cost to do that. Lean on the rep, especially if he knows what he's talking about. He will be a critical member of your team.
Finally, jump on the Talk Car Wash facebook group and reach out to people in Charleston. The group has thousands of car wash owners and operators. Someone there will know something that you will want to know.
Good Luck!