Ayer, Billerica, Hudson, Maynard - those are going to be your best bet for cash flow in Middlesex County.

Hey give poor homeowner a second chance etc etc .

The settl4ement was private, but I bet it was at least $400k knowing the attorney's involved, maybe more. 

These include accidents (I recently had 2 units destroyed by fire), natural disasters (I have had two units each hit by 2 hurricanes), property management turn over (they sell or stop being as good, I have experienced both), or turn over of team (I am turning over at handyman and I suspect plumber), or just wear n tear of property (slab leak in poor location, sewer pipes rotted out (one unit this month), roof, foundation issue, etc.).  

Under the same scenario, a GP with a fixed percent return would probably make the choice to sell and take profits because they would see the 20/80 decision as a bad bet. 

The departing tenant says she will continue paying rent, but I wouldn't bet on it.

If your goal is to offset your mortgage with a house hack then Lowell would be a good bet, but if you're looking for a longer term appreciation play then the areas closer to the city would be better, although the offset from cashflow may be lower than what you're looking for.A good step for you would be to dig into specific areas more as @Lien Vuong noted above.

Best bet is likely (not absolute - everyone's situation is different) is to look at a national lender or broker that has options in lots of states - its a great development versus 20 odd years ago, with technology, internet, places like this - can have access to many more options than before (local broker or lender thats it)

It can run long, it can run expensive, it can be done poorly, it can not be done at all, you can be ripped off. 

Your best bet are still the Townships - they have the best schools in the county with the least restrictions.