Welcome to BiggerPockets!

You may want to consider purchasing one or two year notes with yield of 6-8%. 

Let’s get those deals!

Whatever I do, I would need to do with a HELOC—effectively financing 100% of the property (down payment and closing financed with HELOC, rest of purchase financed with a mortgage). 

I grew up in RI but moved to Boston for college and have been in the Boston area for 15 years.

Since you're 6-12 months away from buying, use this time to build investor relationships, master deal analysis, and secure financing.

I need to know how to do a "Rent to Own" properly as part of the strategy for this unit. 

If you have worked with a company that handled everything for you- vs making your own deal/team/etc, I'd love to hear your thoughts.  

@Nick WardWelcome to the community.

Modular, on the other hand, which I also deal with, you can do a mod and put it in the city limits but would cost as much or more as a stick built, more if you go with a two story.You're much better off going with an existing duplex