Almost nothing penciled out and I wasn't interested in buying properties with negative cash or banking on the possibility of future appreciation.So I resisted buying bad deals and pretty much sat on the sidelines from 2005 to 2009 and continued to build up cash, contacts, my credit score, and my RE knowledge/education. 

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

Key factors include your experience level, credit score (600+), down payment (10-30%), property appraisal and ARV (65-70% of ARV), project scope and budget, exit strategy, cash reserves, market conditions, and lien position.

: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.

. - 35.7% is an incredible increase in a year.  

I have a good income and credit score.

I am happy to take a look and tell you what you should be getting (there are some pieces of the puzzle missing such as credit score, etc.) 

They don’t understand that their 580 credit score will severely limit their options6.

A heloc is an open end debt that hurts your DTI and can decrease fico scores.

For your investment property loans, if they are done in an LLC, most of them don't report to your personal FICO Score, however you can ask the lender before you get a loan to verify.