I'm exploring air filters for tenants, a friend mentioned it being a benefit with their lease.

@Matt Smith - as @Christopher S. mentioned you cannot use an ADU unit in Chicago as an AirBNB as the city's intent with the program is to provide additional affordable housing. 

As part of my strategy, I would like to build an master bath/bed extension, with its own entrance, to the three bedroom, two bath home mentioned in a different post.A 20'x15' (could be smaller) structure that can accomodate the bedroom and the bathroom.

I would also mention that once you have invested in Utah, I recommend the Utah Rental Housing Association as a resource.

He has already mentioned that he is willing to act as the bank, interest-free, which is a great opportunity.

Everything I am learning about mentions Local.

As mentioned by @Chris Mason, Bridge Loans: These short-term loans can provide immediate financing until more traditional financing becomes available or until the property stabilizes and becomes more attractive to lenders.Interest rates and stipulations may be a pain.Out of the box here and requires a few more research, SBA Loans: Small Business Administration loans can be an option, especially the SBA 7(a) loan program, which can be used for hotel acquisitions.I heard someone  mention - Mezzanine Financing: it involves a combination of debt and equity financing, which might appeal to some investors looking for higher returns.

Like you mentioned, the planter boxes were already there, the gravel walkway was already there, worn out, in need of work.

Not sure if it was mentioned here already, but another creative alternative is to purchase a SFH that has a casita. 

As Jay mentioned seem like a fake lender or a broker who has no clue what they are doing.