I'm actually about to replace the front door on my apartment building and was probably going to go with Fiberglass. 

Obviously if it was more than like an 8 unit building it might be best to hire a management company.

I'm considering purchasing a plot of land in Burdett, NY and building multiple tiny homes on the property.

Adding value to your property like an ADU/DADU but those also cost money to build so overall you still wont cashflow.

In terms of mitigating your risk - particularly if you buy a condo - consider a newer building, steel or masonry construction, a unit on Floor 3 or above where Flood insurance wouldn't be necessary, larger buildings where costs are spread amongst more owners, and buildings with a financially healthy HOA (track record of good management, reserves in the budget to cover large capital expenditures, etc).

To provide clarity about my situation, I am currently job transitioning and would greatly appreciate insight on building a portfolio with income limitations.

But I'd also like something a bit less intensive than a BRRRR to build confidence in my first deal, even if the cashflow is mediocre. 

Is building an ADU a good idea right now, or ever in this area?

I have been looking for opportunities for a while now for a fix and flip to start building capital to eventually build up a rental portfolio.

As I need to register this business with the state and county, should I need to register LLC as "531110 - Lessors of residential buildings and dwellings" (NIACS code) or something else?