Kind of late to the game in terms of response as I just joined BP, but I figured I may be able to provide some helpful information none the less.
Speaking from the perspective of a former Boston real estate agent that specialized in rentals/income property, I'd highly advise speaking with managers or agents with good track-records at some local rental companies, mainly for 3 reasons.
1) Unlike MLS for sales, no central database for rentals exists, period. I'm aware there were attempts with some websites to centralize listings (not sure if I'm allowed to name them on this post), but they were largely ineffective. They only picked up the over-priced listings from the larger landlords/property management companies, not the multitudes of smaller landlords which more accurately gauge market price. It's therefore imperative to find a company that not only specializes in rentals, but also one with a large and accurate database to get accurate information that would attribute to your return on investment.
2) Rental agents deal with rentals every day of their week. That is their specialty. The successful ones know their markets extremely well and can tell you about rent trends such as where rent is increasing the most, what/where is becoming popular, as well as what kinds of tenants you'd get in a given area, and what those tenants are looking for in a place as far as amenities or layouts.
3) Most importantly, rental agents are constantly talking with landlords regarding showings, availability, price drops, tenant screenings, etc... They know if a landlord may be looking to sell, upgrade, or whatever which could mean a cash flow opportunity for you. I realize sales agents do this too, but rental agents (the good ones) typically do this at a much larger scale and as a result (hopefully) have better working relationships with these landlords.
As an example I know/knew a landlord that told me he was willing to unload a property for a certain amount, that I knew would rent at about a 7-8% cap rate. I knew another landlord that (through his son's rental company) found a 3-fam property, bought it for 1.2mil, sunk another 200k in renovations, and rented it for $14k per month.
My word of caution however, is that you need to find a rental company and rental agent that's worth his/her salt. This industry is over-saturated with people, often college students, trying to make a quick-buck. Do your due diligence and find a company with tons of rental listings first, then find an agent in that company who makes a living off rentals full-time, and who has been around for awhile (really anyone who's been around for more than a couple years).
With all this said, my personal recommendation (though somewhat dated as I've been out of the Boston area for a couple years) would be to look into the Fort Hill area (Roxbury) and East Boston. From my understanding, Fort Hill is supposed to be the next "Mission Hill" given its proximity to several colleges.
East Boston seems to be the next "South Boston" where many young professionals are going. Jeffries Point is already increasing in value, and it's likely the surrounding areas will see an increase in value as well. Jamaica Plain may also be a good place to look into too as it's convenient/popular for both young professionals and students alike, though it may be too late to get into that market as far as investment, unless you manage to find a really good deal.
Hope this information helps. I wish you the best of luck!
