I plan to operate them via a reputed 3rd party management company which has extensive experience.

Anyone know the answer or able to recommend a reputable lender?  

I also feel like it's a reputable company that I can trust.

I am reaching out to ask if anyone has explored avenues of building or remodeling a structure into a Cohousing situation here in SD?

You could explore "USPS Street Addressing Service," the UPS Store has one, or you could use one of the virtual office services that have a physical location.

We feel that we have been given a great opportunity to get started investing in real estate with this chunk of money, but what other options, if any, can we explore so we don't need to sell our brand new home (which took 15months to build!). 

Hey all,I've been renting in Brighton, MA for the past 4ish years and am looking to purchase a small multi-family in Medford sometime in the coming year. Was curious if anyone has experience investing in this area or ...

Personally I would only go with reputable conventional financing.

What state should I consider and explore? 

Not trying to deter you at all, this is an awesome niche that I'd love to explore too, but I think to do so you'd need to be buying in a very expensive area which allows the ADU and where rents/value added support the cost of construction.