For me A or B grade means that you can get a tenant with a 700 plus credit score with an income well above the minimum threshold. 

Unlike traditional banks, which focus heavily on personal credit and income, portfolio lenders prioritize the cash flow of your property.

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

I have hired a revenue management and listing optimization company that tracks all of this for me on a daily basis.

With expert guidance, tailored training, and a supportive community that will align you with the tools you need to fast-track your success and avoid common pitfalls.

The reason being I have to build up my credit score before I can get approved for a loan. 

The seller doesn’t need to know that the underwriters may give you more of a hassle than other buyers.Or do you have lines of credit or a private lender so you can make a “cash” offer then try to get the financing through before close or at worst just do a cash out refi after close?

Screening tenants well and then a security deposit or credit card hold process in place will limit liability.

If your rate is low on the 5 unit, I might consider the possibility of a line of credit for my equity instead of an entire new mortgage.  

Everything he teaches about personal finances (budgeting, car loans, credit cards, etc.) is spot on.