So currently the neighbors grey water mostly goes in my driveway, and there appears to be what used to be a toilet that might be leaking sewage in the basement.The floor drains have collection of water.   

Well I think you should only increase the rent if the current rents are under market value for the condition of the units.

A cash out refinance to buy more property would put a substantial dent in cash flow with current interest rates.

Subject To deals are RARE, but being familiar with the concept can allow you to get a deal done quickly, with a real plan to get the seller's note paid as soon as is practical, but immediately if it becomes necessary.

Did your current property manager place this tenant? 

I'd love to put you in contact with these people and start marketing your services to them, I have done this maybe 40 times now with my current lenders and i typically just add my payment into the HUD and the borrower pays it, this service wouldn't cost you anything.

Here is an excerpt: Standard of Practice 1-7 When acting as listing brokers, REALTORS® shall continue to submit to the seller/landlord all offers and counter-offers until closing or execution of a lease unless the seller/landlord has waived this obligation in writing.

Do you think it's a good money investment considering current crushing market and if you consider this as a safe investment. 

Depends on your local rules and regulations, but in the state of Virginia such a practice is perfectly lawful.PS: Do not take model legal language and incorporate it into your contract unless such language comes from an attorney. 

(He has not taken a salary from the nonprofit in 20 years- but the proceeds go to a cause he cares deeply about)  Currently, my sibling and I own 40% of the building, and the nonprofit owns 60%.