This would be considered residential and we'd only consider it a 4-unit, so typical conventional lending guidelines would apply. 

Typically when someone is talking about wholesaling properties, they are referring to assigning contracts, which is why I stated:  "Legally speaking you cannot publicly market a property for sale that you do not own without a real estate license.

Foreclosure auction is typically cash only.

rentometer is a good option for rental comps, also speaking to a local property manager may give you a good idea of typical rents. 

So owners aren't typically open to financing deals at the same rates banks are loaning at?

Typically I'm looking to set this up in the Braintree/Brockton/Bridgewater area. 

At a quick glance this is what I am seeing: You’re putting in $5,100 Your gross rent is $1,650 If you take out 10% for Property Manager and also remove 5% for Vacancy (2 months every 3 years) then you’re left with about $27.50 per month, which sounds really ugly but when you look at COC you’re actually getting 6.5% which is pretty typical to what I have seen in this market with 30year loans.

What type of tenants typically live in places like that and are they easy to find? 

My only exit strategy would seem to be resale with appreciation bc at $420K purchase price (new build in nicer area), I'd never generate $4200 monthly rent (typically around $1800-1900 here).

You would typically want to have your contractor walk the property with you before you make your offer so you can factor the rehab estimate into your analysis and then you would have an inspection done immediately after you get the property under contract during your contingency period to make sure there's no other issues.