Closing on the duplex first and then finding a single-family rental works well. 

My son currently lives in the townhouse b/c he is going to school and works in the area.Since he is young and starting out in life, I would like for him to only cover the basic costs (taxes, HOA, utilities) just enough for me to break even (I believe that will put him slightly little lower than market rent rates). 

Or buy a single family and house hack bedrooms or a portion of that property. 

I net about 3.5-5k a month(depending on airbnb) after all expenses including my primary residence and utilities there.I have built up a good amount of equity in the houses due to the renos I've done mostly myself and also from buying low during covid.

When you say 'legal one family', do you mean that it is zoned for single family residential?

Meanwhile pay little or no taxes utilizing depreciation.

Adding a single small unit is very expensive development.  

Targeting single family/duplex/triplex/quadplex with 3 beds and 2 baths per unit, and $300K or less.

The most successful people that I have heard on podcasts and met in real estate know that it is all about getting a single here and there and then going to get a homerun while doing those singles.

I intend to get a single family residence, add an ADU, live myself in the residence and rent out the ADU as a studio apartment long term.I have checked my city's ADU laws and long term renting is permitted.Question : can I obtain the REP status while maintaining the ADU while it is in my primary residence?