The summary is, although RS3 zoning is primarily for single family homes due to the code restrictions, when the building already has a second unit established legally, it’s grandfathered in.

You are not responsible for repairs, you should be listed as an additional insured under an insurance policy in their names (establish minimum coverage in your contract) You are not liable for anything, you don't own the property anymore.

That means you establish the financial criteria you "need", not "want", based on a plan. 

Buying for appreciation is a gamble.

New investor moving down in philly, and hopefully to establish banking relationships.Thank YouAbou

Many units in the area were converted before zoning was created ~1950 so if you can establish that the unit existed continuously from that time then you would have a legal duplex.

I know people do it, so it can be done, but especially in a multiple replacement property situation, it's a gamble!

Condos in established neighborhoods. 

IIRC, our initial balloon was in 7 years, but I worked on refi'ing to get out of the ballon pretty much as soon as our business was sufficiently established to have a chance of convincing a bank to get us a better loan.

Either way, landlords are able to establish rules related to the care and presence of animals kept on the property.