This includes credit, criminal, and rental history checks for all parties.

Additionally, since my parents are nearing the retirement age, I'd like to prioritize low-debt, high-cashflow deals.

You can go as low as 2.5x in some cases but NJ tends to produce enough traffic/applicants where 3x should be minimum unless property is over priced.

Looking at this mathematically, if you assume a 50% non-renewal rate (industry standard) and use the low end of the range of $5,000 per non-renewal, there is a loss of $50,000 per year on a 20 unit property.

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

A cash out refi should take 2-3 weeks with most lenders, and you should have only an appraisal a credit report fee as upfront costs. 

The current rate is extremely low.

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

My W2 income is very high (+$500k), excellent credit, but I hate my job and I want to leave within a year or two. 

-Max LTARV is typically 75% -Is a 12-24 Month Interest-only loan with a balloon payment - Minimum credit score is typically 600 -Interest on the Rehab is usually paid per the total drawn amount.