If you buy the property and there are environmental contaminants they would become your problem and it is not cheap to have them re mediated. 

It's not just contamination of your potential site you want to own it is what is AROUND your site that could also get contaminated and leach onto your property.Example a small business gas station owner goes out of business and the tanks leak and contaminate your site next door.

What if it's a triplex that needs three of a lot of expensive things, asbestos abatement, discover there is soil contamination because it used to have a small gas pump and holding tank under the driveway from when the 3rd unit used to be the driver's quarters in the early 20th century and it's leaking into the earth (I've actually come across that) and it totals way more than $65k?

If the tanks leaked, remediation and cleanup will have to be done, and the property readings will have to be under maximum allowable percentages of contaminates for a certain period of time before the property can be re utilized.As to the asking price, either the seller doesn’t really want to sell, the seller wants to see if he can find a sucker; or possibly the value of the land for future development is much more valuable than the existing use.But assuming that the latter is not the case, what is it that attracts you to a deal priced over twice market value with a possible environmental problem to boot?

If you don’t test for it and know the exact levels of the contaminants, you don’t know which format is required for disposal. 

Or, should I make a lower offer, and buy as-is with the only stipulation being that the oil tank didn’t contaminate the soil.

With industrial or properties involving hazardous materials (gas stations, medical offices) is the risk of environmental contamination usually covered by insurance (which the tenant would be paying for)?  

It depends how much soil is contaminated.

Another vote for siding due to the possibility of lead paint chips contaminating the soil at the base of the house. 

Remember when we went out of our way to make laws about flavored vaping because of some contaminated weed vapors?