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

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.?

We had one young lady, good job, good credit, good rental history but she had poor taste in men.

I'm looking at purchasing a rental in North Carolina, I have Excellent credit (800+) and money for the down payment..

With solid credit and property have you considered just going the conventional route? 

If you have a seller finance note for $2k/mo and make $4k/mo then you have a DTI of 50%...it won't be on your credit report, but is a debt that needs to be disclosed.

I invest in mid to upper-income areas where good credit scores and high earners aren't hard to find. 

Just gotta be real with them :D Let them know you will try your best to have seller cover all of some of the closing costs.Can also always offer above asking price and ask for the difference as seller credits

If you intend to pull money out long term, I would do a 30 year fixed rate 1st mortgage as it will offer the lowest interest rate out of all lending options on the market.