The credit score and income will help you determine if they can pay and will likely pay on timeThe landlord references, eviction history, and criminal history can help you determine how they will treat you and the property.none of it is full proof, but the more info you have the better.

All you need is a single place to track tenant information, payment history, maintenance, and other things.

Verifying last 2 years of rental history very important!

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

For Conventional, you can only use 75% of the gross rent until you have Sched E history for the rental for a net rental income analysis. 

Verifying last 2 years of rental history very important!

I'm an independent agent here in AR and I write a lot of rental property, with ~6 preferred markets and if the properties have some issues, age, claim history etc.

Even if a score doesn't include a history of everything, if it has enough data it should be useful.

Most (not all) HELOCs require that the property is titled to an individual (not LLC) and are full doc, meaning your employment history, income, DTI etc will be factors.

(Auctions and foreclosures can also provide great leads.)With $20K, hard money loans are a practical option, as they emphasize a property’s ARV over your financial history.