My concern is since I don't have HELOC setup yet and am planning to get the 80% loan in 4 months, would opening the HELOC now lower my credit score meaningfully to make the loan more costly ?

I have not been able to realize any meaningful profit on the arrangement.

We need a whole lot more information to give you any meaningful advice.How much is your house worth?

Banks are lending anywhere from 4 to 4.75 on 25 yr commercial.If the deal is terrific, then take his loan, reposition the property and refi in the near future to pay him off and possibly extract some capital for yourself.I always like to hear seller financing deals.  

I may use it to calculate return on my SFRs (although I prefer ROI as more meaningful in the short term and IRR in the long term).  

Honestly, I have only defined cash flow / income requirements as 'maximum' at this point and will need to work on putting a meaningful number to it.

I am guessing what is meaningful for you now is deals and experience.

What is that number and how meaningful is that?  

Thanks you for all of your comments you guys are giving me some great meaningful advice for us.  

If you are regularly or in any meaningful fashion giving free labor (aka sweat equity) to the IRA, that would be a clear violation and something that could draw the ire of the IRS in the event of an audit.