Selling a property in the first 7ish years with a mortgage is not great because the strong majority of your mortgage payments are interest and the principal balance doesn't go down much. 

During the draw period you are paying 1% of whatever your balance is and it is lowering your principal and interest with each payment.

Has anyone acquire a HELOC, where the payments during the draw period were principal plus interest?

Some are interest only, while others require both principal and interest.

You could just deduct the payment from the cash flow (assuming it's both principal and interest) after you run the report, while reflecting a 25% down payment within the calculator.Also, while this will impact your cash on cash return, your cap rate will be unaffected as cap rate disregards financing.

So everything in surplus goes to additional principal payments in the contracts.

An installment sale to a dealer, which results in:a) deferral of capital gains tax (for 30 years);b) an installment note from the dealer, which pays the seller interest-only payments for 30 years, and ending in a balloon payment, at which time the principal is repaid to the seller by the dealer and the capital gain is recognized.2.

Net Operating Income2nd is cash on cash3rd is cash on cash + principal from debt service

How do we take fluctuating rates of HELOCS into account... the thing about HELOCs is that there is risk in rates rising, but there is also more ability to affect your monthly payment and therefore increase your cashflow by paying down principal... which you can't do with a fixed mortgage.What are your thoughts for what points to consider and what levers to pull when comparing the two options when its a HELOC you're coming to a deal with?

This you could probably get away with not reporting as income, and could use it to pay down your principal faster or save towards additional investment properties.