My intention is to not initially increase the rent, make some improvements to the efficiency of their HVAC systems (go from ancient oil to high-efficiency gas) and, once I have the work done, increase the rent to compensate.

Can I keep the sale price the original offered amount, and get a cash concession at closing, so I'm essentially walking away from the closing with the money in hand to pay for the repairs? 

What is the process if removed a underground oil tank and the soil is contaminated?

Is anyone noticing that oil is crashing? 

In this scenario, sure the owner makes a bunch for essentially financing it for you but you're not plunking 25% down up front and the paying big interest only payments each month.

Besides, if I can borrow everything (25% from line of credit and 75% from portfolio lender) for the MF, that is essentially "no money down" and my return in infinite. 

You could offer the tenants "cash for keys", essentially pay them to leave early before their lease is up. 

Have them recommend a routine maintenance schedule, advice you on existing issues, and give you an idea of what might happen in the future based on the systems you have.On wells, you will essentially follow the same steps but you will be using a plumber.  

When you flip a house you are essentially a house dealer.

You could, essentially, live for free.