With all that, it's the duration of things that concern me.

Look at all the numbers for the duration of how long you will own it, not just the month to month cash flow.

I know with most, the basic structure is funding a percentage, usually 65-80%, of project cost, with interest only payments during the loan duration, with a balloon payment at the end.

Yes, Angela @Angela Dulin I want to have flexibility for any rental duration (Airbnb style, month, 3 months - all of it), without restrictions imposed by HOA or local laws.You are saying I need to receive a permit from the Metro Codes Department prior to listing property for Airbnb, or any rental in Nashville? 

Unfortunately I think the part that I read that's a deal killer is going to be the expected duration to own the property.

.- Loan Duration is 3 months minimum and 30yrs maximum.- No prepayment penalty attached to my loan program.- Loan interest rate is 6% minimum and 7% maximum depending on the loan amount and duration.- But then they say, "we charge loan origination fee and is the only fee required before getting you funded, It's 100% refundable".

I therefore choose HELOC only when I am expecting to have the loan for short durations such as a purchase with a planned refinance after performing a value add.  

I currently am a construction manager and my company moves me every 1-3 years depending on the duration of the job. 

The required duration of consecutive current payments varies between lenders

The only caveat is the inability to purchase a primary for 5 years, so you would have to buy strictly investment properties for the duration of that time.