I have done Phase I, II & III's, but all industrial properties, no residential.
First, actual data will only cost about $200 for an initial sample and is worth more than expert opinions/conjectures.
Second, in my part of the country, the EPA or local administering agency isn't interested in anything minor, e.g., a $100K cleanup. Prosecutors won't be interested since there was probably no criminal intent (see 'Fourth').
Third, one should first verify the ability of the responsible party to pay for the results of any EPA interest. One case, it cost a client more than $100K to verify to the EPA that there wasn't contamination, and there was no chance of reimbursement from past tenants, anyway.
Fourth, (perhaps a worthless opinion) I doubt the previous owner felt that he was doing anything wrong. He was doing this to his own property, not midnight dumping. Perhaps building up a low spot [let's not even think about wetlands]. He may even have been loosening up some clay soil.