The property taxes are reasonable and so the insurance (let's say $3000 for both or maybe a bit less)- Kansas (Lee Summit [MO] or Overland Park [KS]) are contenders as well where buying property is more affordable BUT property taxes and insurance, combined are $6K'ish, with better ranking schools. 

And if those people say no, for any number of reasons, then you ask them if they would introduce you to one of their connections and ask for very specific reasons they are not open to investing with you.  

They tell you nothing of true value, and will lie to you

A traditional bank would not be interested in lending for that reason.  

So one of the reasons I like managing my own properties is I can vet the tenants myself, not only screening criteria but in one of my units I had someone who contacted me, and frankly her credit score was below my criteria but she reached out told me she has two kids and really loved the unit and she’d be sure to make the payments, I decided I’d at least show her the unit, and when I met her she was super sweet & I could just tell she would take good care of the place and really prioritize making her payments, and I was right even the few times she’s been late she’s apologized and has never missed a month of payment, so I’d self manage your properties in the future & I think if you know how to both screen & read people you’ll do better than with a property manager.

The reasoning is they say that the registration is based on unit number not ownership of the building - which I do not understand at all. 

Is there a particular reason you're only considering those three locations?

Are they friendly, knowledgeable, if they don't answer, do they call back in a reasonable time? 

I see some units that cash flow but there must be a reason.

(from study guide)It is important to know the years each act was implemented.Know the reasons, regulations, and ways a landlord can increase rent.Dates for HVAC system tests, turn on/off dates, and temperatures they must maintain.Hot water must maintain 120 degrees.A requirement for housing is to have an exterior LOCKING door.When an owner buys a building they are also buying the lease and must honor the lease.I would suggest to read the "What you should know about Rent Control in the District of Columbia" link on the DC.gov website as well as the study guide from pearson vue inside and out.