Sterling,Each lender is different on how they calculate income and also what overlays they have that can cause a bad income calc. 

I have a lot of bank accounts and credit cards and it seems impossible to keep track of everything.

They have no credit history, so you have no idea how responsible they are financially.

Otherwise, if the numbers make sense and your credit is good, it should be easy to find a lender.

I can't remember the cost, not too bad, but no phone callsI sent out 1,000 mailers (letters) I designed in envelopes a week for a couple of years and made a ton of money buying properties using "creative finance" some call subto.

Yes, there are many out there that do it and nothing bad has ever happened, and I say kudos to that, I just would never do that.

Some will be ok in 2nd position and some will not and force you to refinance out the first - but those are options that are probably cheaper than credit cards 

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

@Helena Goyvaerts rate for a 30-year fix would be at least 8.5% with some points (1% of loan amount).Most DSR lenders have a minimum credit score requirement from the applicant.

I usually try to get 10k but sometimes its not possible and I would prefer to make something and give the investor a good deal rather than make nothing trying to move a bad deal.