You need same day cash typically, no loans, no credit. 

I am looking to refinance an existing HELOC to increase the line of credit.

Sounds like you are on the right track doing research on different markets from the Midwest to Texas. 

If the goal is to work with a well-priced quality lender, narrow your search to DSCR lenders that have solid reviews and a track record of closing.

I recently explored several top mortgage companies to hang my NMLS mortgage license with and if you have decent credit and a down payment, banks are finding ways to get you qualified.

If you are lookign to occupy the property as a primary, then you'd need to look at the conventional (Fannie HomeStlye Reno loan for example) or FHA (203k loan for example).As for the takeout, again so long as the ARV is strong enough to support it, you have credit and income to qualify (if you go the Conventional DTI route as opposed to DSCR), you should be able to refinance into a long-term loan no problem.Feel free to DM me if you'd like to chat more on this!

Sounds like what you're looking for is a line of credit on this rental property.

His credit score is 620.

If you are a good rehabber and/or have good assets and credit, you might be able to get hard money loans to purchase and rehab and resale.   

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