It might make sense to use 50% in your underwriting just to be safe and if you end up beating that, your return will be better. 

I heard on a couple of the BP podcasts that they setup a Simply Safe security system. 

The 2 bed 1 bath seems a more wider based safe choice .

There are some "safe harbors" that may or may not apply to your situation.

The long-term goal here is that if we get 4 of these units free and clear then we've got this primary mortgage covered and I'd feel safe turning to investing as our only income, so the faster I can get these paid off the closer we are to that goal.

If anyone has a referral for an honest contractor who is licensed in Massachusetts for deleading and/or a company that does vinyl siding and is certified "lead safe" I would really appreciate if you could pass that long.

Looking to start off small and safe .

I'd invest a good chunk in a safe index fund type of investment.

We chucked the appliances where their nest was (dishwasher) treated twice (both the infected unit and an additional unit not infected but just to be safe) and within a month they were gone. 

I would also say that as the owner of the property, you have to obligation to ensure that both duplex tenants have equal right to the property and that the space is used in a way that allows safe entry and exit.