I can take up to $200+K of HELOC at approximately 5% APR, and i'm thinking about using some of that to invest in properties.  

I'm wondering when do i pay the back payments? do I pay at the closing ?

I've invested in/renovated a number of single family properties in the suburbs of Philadelphia (Mainline), with values ranging from $400k-$1.3m and decided to tackle a significantly larger project 6 months ago; comps were incredible with high sale prices and very low days-on-market, it looked like a home run.The loan is under an LLC but with a personal guarantee; the renovations are complete and the house is beautiful, but the market has been unbelievably slow compared to the previous 3 years, on which our financials were based. 

If this is a single family...you should have 1 month vacant every 3 years or so.Your interest seems low, but maybe thats because you are paying points.

Also for the cap rate question you asked, I wont tell you what kind of investments to make, but I personally would not take low/no cash flow property on the hopes it will appreciate.

Are there special financing sources that could make it worthwhile to set aside some of the residential units for low income housing or put solar on the rooftops or . . . ?

I might have an investor that will go as low as 600 on that as well?

What else do I need to think about, and what would a good “low ball” offer be on this?  

Other than "as low as possible" or "as high as possible."

As for the other neighbors are all the rentals low priced or are neighborhood owner occupants going to cause you issues?