This way you could get into another house/duplex/triplex/fourplex for low money down (3.5-5% down), live there for a year, rinse and repeat.

Here are some things to consider; USDA loans were created for low income rural housing, so not everyone house is eligible for a USDA loan.

However, why would you want to enter in such a low margin business?

LVR restrictions on any property that is not your home, very low yields, high building costs, median house price in AKL that is over $850k.No building depreciation, and government that is introducing pro tenants laws... 

I think your expense estimate is very low.

You can get them at Lowe's for $15-20.

In my opinion, $1k additional per unit totaling $3k is rather low considering the additional liability the Architect will incur. 

I have seen multiple properties in Puerto Rico that I am interested in due to low cost.

I had started low.

The work you mentioned is absolutely negligible and you will open a real pandora's box bringing inspectors into your home and start an endless stream of trouble.