My goal is to understand the current demand and rental potential for ADUs in this part of the city, as well as the costs involved in building.

The helical piers system would cost $25k but is a permanent solution.

.#8 I would consider your inspection cost a very cheap crash course in buying long distance investment properties. 

A couple years later, repairs, vacancy and turnover costs and you've actually lost money every month and you've decided to sell and buy in a more desirable area, but the property hasn't appreaciated, you have to fix it up to sell and incur transactional costs and you lose even more money. 

The two existing LTR units (on my family’s property) were built 5–8 years ago at a cost of about $80,000 per unit, so I’m estimating a 20% increase in costs for the new builds.My plan is to use the equity in the land as the down payment for the construction loan, which I expect will make closing costs the primary upfront expense.

To manage upfront costs on fix-and-flip projects, many investors use a combination of strategies.

After my P.M. costs I collect $940 or so/month. 

Travel costs to look at other properties.If you travel but have not identified the property that you want to buy yet, the cost is considered an investigating cost.

Another note on construction cost, going vertical is very expensive now and would require have a great connect with a developer and a builder in the area.See what rents are in the area and run the numbers to ensure the rents will cover your costs and give you a solid return—using the 1% rule can help.

Your estimate seems low, but even at that cost, it will not add that value.