Generally, this means commercial properties with very good credit-worthy tenants signed on a very long term lease. 

What is a fair/typical % to apply as credit toward purchase price?

When you partner with someone, the money they put into the property isn’t counted against you as debt on your credit score which can have a negative effect on your ability to get loans etc. that is one of the benefits of partnering with others.

The gap is usually due to a few % taken by their affiliate company for the credit card clearance.

My credit score is 830 and I have NO other debts - just my current mortgage.

In terms on a HELOC or a Line of Credit, what's the best starting point for looking in to this?

If your asset has appreciated well enough, you can take a line of credit or perform a cash out refinance.

Large cash and large credit line. 

I believe Fannie/Freddie guidelines are that only the current balance/payment is counted towards DTI, while many lenders will have more stringent guidelines that could possibly count up to the maximum payment if the line of credit is fully utilized. 

It checks credit, criminal and evictions for $26.00 and then gives you a pass, fail or conditional with larger deposit.