If the septic is covered by a slab of concrete, how will they ever clean it out? 

If that's the case, you can't do any renovations until you have it professionally deleaded or it would be flagged for Unautohrized Deleading and you can get fined and the property would never be able to get a Lead Certificate, only a letter of Environmental Protection that states you did illegal work, got caught, then had to pay a professional to clean up the mess.

I suggest you read the Bigger Pockets book written by David Greene "Long-Distance Real Estate Investing". 

I have a 750 credit score with a clean record.

We honestly felt bad that she had paid”professionals” to clean carpets and do a move out/in cleaning.  

Spend whatever it takes to get the correct respirator, prepare thoroughly, and clean up thoroughly(bought a new a shop vac just for this, with the special drywall dust bag and also a HEPA filter).

I was green and unsuspecting of his call and was caught off guard.

Basically if she didn’t leave they had been given the green light to arrest for trespassing.

One other negative, and I don't know if this is true for all because I only have one, it is designed to hook up to a hose bib for cleaning.

Possibilities are endless.... housing, a farm, sell the timber, trailer park, subdivide and re-sell, explore/mine resources,  hold and sell later... thanks. yes, its currently zoned as 'green' which i believe is agricultural use (i'll obviously need to do a lot more research)...might change in the future due to its proximity to the city center.