I havent seen these get approved when the total portfolio was less than $20m or so because banks usually wont go over 40-50% LTV on the portfolio and it takes a lot to underwrite these.

Usually, a default on the 1st is a violation of a covenant of the2nd so a default of the 2nd is triggered even if the borrower is current on the 2nd lien.  

Be sure to learn specifically about Philly - the laws here are very different than most of the country and even from the laws of PA.

Condos are usually a lower price entry point, so they suit a lot of newer investors, but certainly not for everyone! 

DSCR loans cost a lot more than conventional loans. 

There are so many variables that putting an average into a spreadsheet isn't going to do much to get you closer to taking action.

Ask the building Dept what they see as issues or problems as they will be the ones to make this a go or no-go.Biggest issues are usually getting to the drain line.

In San Diego the typical SFH investor purchase rent ratio is closer to 0.5% than what you depict and has not gotten worse at the same rate as you indicate for LA (it was not much better than 0.5% 15 years ago), but has gotten worse.  

Buy well and it will be cheaper than owning/living in your own SFR.

The local laws are usually very clear on how a former resident can obtain possession of their property and trespassing unsupervised is usually not legal.