This may need to be taken with a grain of salt, but I have looked each up on Trustpilot and included the scores below.

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

5+ units DSCR or commercial mortgages. each one is used for different things. really matters purchase price. location of property and your physical location. dscr looks at the property and your credit score. commercial bank looks at you and the property. tax returns, w2s DTI. then the property needs to cash flow enough for them. dscr is a bit more lax but higher rate.

I've been following the discussions here and have found the insights incredibly valuable.

I recall seeing a recent post from someone else in a situation like this, he even paid the collection but score is still too low to apply for a mortgage.Certainly speak to an attorney to double check your options.

This field offers incredible opportunities for growth and learning, and it's great that you're eager to master it.

The VA loan is the greatest loan product available and can be an incredible way to start your RE journey (that is what I did personally). 

I generally don't set a number I look at credit history for lower scores if it is a medical issue I will view differently than if you just are overburdened by debt and just not paying.  

They’re incredible tenants and the husband is a plumber so maybe they just don’t complain though.

:Class A Properties:Cashflow vs Appreciation: Typically, 3-5 years for positive cashflow, but you get highest relative rent & value appreciation.Vacancy Est: Historically 10%, 5% the more recent norm.Tenant Pool: Majority will have FICO scores of 680+, zero evictions in last 7 years.Class B Properties:Cashflow vs Appreciation: Typically, decent amount of relative rent & value appreciation.Vacancy Est: Historically 10%, 5% should be applied only if proper research done to support.Tenant Pool: Majority will have FICO scores of 620-680, some blemishes, but should have no evictions in last 5 yearsClass C Properties:Cashflow vs Appreciation: Typically, high cashflow and at the lower end of relative rent & value appreciation.