If I had to guess it might require revision and re-file of prior year returns. 

It also requires a lot more capital with the potential of using hard money loans.

A LOT of Cities require it and have minimum specs for the windows.

Get something simple, but durable and don't get anything that requires a water connection.

There are a lot of people here on BP that can mentor you, some may do it for free, others will likely require you invest in their program.

This can cause issues with the re-installation as there may not be framing next to the window until after the weight pockets which could cause you to upsize the window leading to interior and exterior trim replacement as well as possible code enforcement issues as they may look at this as a change to the structure and require a structural inspection, even though you're still within the existing framing rough opening.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

I'm a college student going into my sophomore year and I just reached the age requirement to take courses where I live!

Non owner occupied and that will require 25% down

Never sign an NDA to go see a property, some New Western franchises required that, not sure if all do, but that is a crazy request and uncommon in the wholesale space.