things missing in your list (everyone manages differently, so take it with a grain of salt)Vacancy rateTurn overbad debt legal fees (evictions as needed)accounting fees for professional tax prepmaintenanceProperty TaxesSales Tax on PM fees (In NM, this is charged, not sure about where you are)Reserves for replacement CAPEX - furnace, water heaters, ac, roof, parking lot, etc.Admin, software, advertising, etc.Cost segregation study for accelerated depreciation captureAs for your loan. 

Your worst-case scenario (not specific to MA) would be having to sue to get them out as a holdover, but this is really reserved for worst-choice scenarios when owners choose the first renter or don't do any due diligence.

A Heloc can never be used as a PITI reserve, or an asset instead its a debt burden and an additional trade line on credit.A Heloc carries a higher risk when it comes to security meaning if you miss a payment even by mistake or for any reason, your scores drop, or your balances start to change your DTI or usage percentage.

Due to high demand, many investors in Austin are finding they need to adjust their approach a bit to make the numbers work.Rehab Budget: Ensure that bids are received from multiple contractors and that adequate reserves are made for cost overruns.

I was looking to flip a few properties first in order to build up a little stack of reserves before starting to purchase for rentals.

How much do you want to stay in the bank when you purchase (consider you will need 15k still in there for reserves on any purchase)?

A hard money loan with 10-15% down may also allow you to keep the Atlanta property, if you have the reserves to do it.

It's the nature of investing in real estate. 

The fact of the matter is you mitigate that risk by doing what the folks above already told you....you bring money to the table and you have money in reserves if things go bad. 

I'm conservative in nature, but am looking for the push to jump in at some point.