So cash flow for a lot of deals might have changed some but not too much as long as you find the right deals.

That said, it is doable as long as you have the time, personality and contacts. 

Hopefully the deal had enough meat on the bone for there to be enough space for you to still be able to successfully refi when you originally planned on as long as interests rates don't make a substantial jump. 

As long as you have money to buy and maintain them, there is no limit.

My specific questions are: 1) how often can landlords increase rent on a month-to-month lease (can they, in theory, increase the rent month over month as long as they're giving notice each time)?

All of these neighborhoods are in high demand because of the direction Philly is heading so as long as your numbers work, go for it.

It does not matter where you start as long as you develop your Core 4.

I understand that several investors won’t deal with on market properties, but their are plenty of investors who will as long as it fits their numbers (most of the time) right? 

I knew by quitting teaching (W2 job - for those out there... keep them as long as you can!)

The structure you are looking at works fine so long as the asset generates sufficient cash to cover expenses; however, as all RE investors know, the unexpected furnace malfunction or roof failure requires injection of outside capital.