It's also unencumbered by debt, purchased in foreclosure, and renovated to serve as a lucrative STR, yielding approximately $115k annually with a net cash flow of around $75k.My immediate 18-month goals include undertaking essential long-term maintenance and capital improvements to the Cape property (~$75K), which should augment the STR income.

Building multiple MFHs as your first step into investing is a big undertaking in my opinion...I know you want something new to lessen the burden of upkeep, but I'd suggest starting with an existing property.

Think both:)ADU: cash out of pocket -$80k, home equity +$400k, +$2200 cashflowFlorida :  -$80k out of pocket, 2 properties,+$3200 cashflow (not including expenses)Building new construction: This is a bigger under taking and a lot more work but could be worth itHopefully this helps, good luck!

Is this a feasible undertaking for myself?

I was fairly lucky with the inspection process and did not have any major undertakings to get everything approved.  

Reconfiguring the layouts to residential AND bringing these old buildings up to code is a big undertaking.)

I plan to undertake an Industrial Flex space project, first timer on any real-estate project .

When we undertake a new build we are really dealing with two purchases and two costs if you will. 

My question is this: If I'm doing a fix and flip and am willing to pay the private lender on their terms, what risk do they undertake?

If I were you, I would make sure that the after repair value would be worth making such a large investment of both time and capital, and that the amount of monthly income you could expect to receive would support the numbers.The project you are looking at seems like a huge undertaking from the pics you included.