The only way to be 100% sure if the soil isn't contaminated is by testing it.

He has offered to have the tank removed, but is unwilling to have the ground tested to confirm there has been no contamination (he plans to "take a look at the soil" after removing the tank, and has offered to let me do the same).

Even if it was "properly" abandoned years ago, the possibility remains of soil contamination below the tank.

If the owner is operating the business and you won't be you will be employing a manager so will lose that profit off the top as an absentee owner.Those machines are expensive to replace so you would need to see how old they are.It's just like an old house or building with deferred maintenance.Everything might work when you buy it but the chance of things stopping to work will happen very soon and cost a bunch of money to fix.You always buy the assets and NOT the liability.I can tell you I have had people come to me AFTER the fact buying partial ownerships into corps etc. and they are stuck.I do not know who advised them to do this but they were an idiot.Now I have had to help them pick up the pieces.With the coin laundry or car wash make sure the building is owned so the lease doesn't keep going up or renegotiate the lease as part of the purchase.Looking at books is fine but owners tend to manipulate the books and not the tax returns.The tax returns are usually more telling but people can lie on those as well but that takes big balls and doesn't happen often.If the coin laundry is older make sure if you own the building there is not galvanized going to the street for the water main.It's very expensive to dig up the road and replace with a copper line.You will also want to have the EPA phase one to make sure there is no contamination of the soils.For competition you can look at applications for permits,businesses and new site plans in the city or county you are buying in.You can also instead of buying outright have a 3 month period of running it with an option to purchase at the end of that 3 months.This way you get to see how it runs and decide if it was everything as presented or they were trying to "dump a dog" on you.If they strongly oppose this they might be hiding something.I can see losing a small amount of option money if you walk away and not exercise the option but that is better than being stuck in a bad property.

I am looking at a deal where the property had an underground oil tank removed and the soil test shows mild contamination.

They tend to do well in more off the path areas where big corporate gas stations ( 7-Eleven, QT, Racetrac, Sheetz, etc.) will not go currently.The big gas stations want 1 to 2 acres minimum with dozens of pumps and a huge grocery on the inside.The smaller gas stations we do not usually buy because even with clean environmental they could go out of business and now on the hook for clean up for the property you own and surrounding properties the contamination leaches onto.

Do you break those out and write off the interest on the balance or does that contaminate the whole HELOC write off eligibility somehow?

My company recently shifted gears disinfecting Covid-19 contaminated areas.  

Cabins are in the once contaminated industrial park north of town.

You need an attorney with experience in dealing with contaminated property purchases.