Christian, this is a tough one to be honest. If your true goal is pure cashflow, then this is going to be tough to achieve. I agree tat you are not accounting for any maintenance and bigger capex (like you said, bathroom remodel etc)--it really adds up quick.
That area is defintely a great rental area, you'll see a lot of grad/young professionals priced out of downtown, but its still going to be tough to meaningfully cashflow. Even if you use your numbers above (which I dont think you will realize), if you pull 200 a month or so, which is 2400 a year, compared to the 40K you have to put down is ~5-6% CoC. Yes you should look at IRR, but I think that tells you a lot of the story.
If you are really looking for cash flow, I think you are going to have get out of the "T" areas--just too competitive honestly. To really get that clean ~300 per unit CF, unless you find an awesome off market deal, I think you'll need to look a bit north (or south)---can be areas like Lynn, Wakefield etc. Not saying you can't money in Dorchester, but cashflow is tough.