The interest rate on a HELOC or Heloan may be better than the personal loan. 

Appraisal fee seems very excessive if this is not more than 4 doors. 

One of my early properties barely broke even, but after a couple of years, rent hikes and expense optimization turned it into a strong performer.For cash flow benchmarks, I usually aim for $200-$300 per unit, but your situation.. house hacking.. offers unique benefits like reduced living expenses and quicker equity buildup.

Sell for more than that and you’ll get the leftover cash after paying off that x% of the loan.

Much better than my STR.

But, these broad market studies are so large, they are almost meaningless, in the real world.Between glut of new supply in many "hot" markets coming online, a continually softening economic outlook, inflation over last couple years making day to day items feel cost prohibitive, many average renters are strapped and seeking lower cost housing.  

Having a duplex gives you far more flexibility to make that move than owning a SFH.  

Flipping is actually more a business than it is investing and you know how much goes into that.

If they can build a larger home and sell it for the same or more rather then building a couple small homes why do it?

It justifies the property selling for greater than just market appreciationI do owner builder rehabs but sometimes use contractors for large work scope or scope beyond expertise.