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

There's a few different ways to focus on a lower rate such as with house hacking and making the investment your primary residence, requesting a significant closing credit from the sellers for standard rate buy-downs (which I've had great success with recently), or utilizing lenders who offer short term rate lowering solutions such as 2-1 rate buy downs (again, ideally paid for by the sellers).

I did find It should result in a 2.5 to 3.5 kw system, net cost around $5k after tax credit.

A line of credit won’t commit you to a monthly payment unless you use the money, so you can avoid paying interest when it’s not in use.

Instead, you could explore pulling equity out through a home equity line of credit (HELOC) or a cash-out refinance.

No social or credit pull needed upfront. 

Private lenders tend to focus more on the value of the asset or project rather than your credit score or financial history.

Then think about writing off your cellphone, your internet, you car, your home office, your tax prep, etc etc etc.  

This includes credit, criminal, and rental history checks for all parties.

A cash out refi should take 2-3 weeks with most lenders, and you should have only an appraisal a credit report fee as upfront costs.