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 ?

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

And unlike many of the cash flow-centric markets @Ali Boone listed, you have the potential for meaningful appreciation if the positive trends in many of these intown neighborhoods continue. 

Hopefully, as I learn and create some connections with you all that will subside.I'm very interested to create meaningful relationships where we both can add value to each other. 

Listen to @Mike Dymski.When the total amount of invested capital gets low, the CoC number becomes less and less meaningful - while it looks great on paper, earning a 47% CoC on $5000 invested gets dominated by other risks and time costs for you. 

Some people do very well in this business extracting every ounce of debt they can obtain.

in mutual funds or syndicated investments its someone else running the show.. when you buy a turn key property ( a home that has fresh rehab and is for rental purposes) your running the show.. its as simple as that.. there is no way to get any kind of meaningful data like what your looking for.the only meaningful data that comes to mind for me and I have been in this space since 2001 .. is really asset or neighborhood quality.. low end neighborhoods generally are tough on out of state investors and will not perform as well as higher end neighborhoods.and what do I mean by that...

If you get on the phone, you will quickly determine who is selling a set of documents and who is going to be there down the road with real, meaningful answers to your questions. 

If you want the full view, you should become an Agent or develop a—um, ‘deep and meaningful relationship’ with one.