Is it credit score?

There are several factors that go into play such as Credit Score and what shows up on the credit report.

is this different than the online version?

A hard money lender can get all the information on you on file before you find a deal regarding your credit score, driver's license, track record etc.

its the Nigerian prince version of lending. 

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

@Cirilo VillarFHA loans, including the FHA 203k, are for qualified non profits or primary residence owner occupant borrowers.Consider the Fannie Mae Homestyle or Freddie Mac CHOICERenovation loan which is the conventional version of the 203k and investors can use it. 

The other two don't have enough data to generate a score because the tradeline driving the 806 score is only reported to the one bureau.

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

To get an abandoned house - like the ones seen by the score throughout the 05 - up to section 8 standards for only a $20K investment (including taxes, insurance and other carrying costs) seems implausible to me.