We do both, pure rentals and personals.

I think this is more like running a business than purely a RE investment. 

The technology that will make this possible will be solar, high efficiency heat pumps, geothermal, batteries, and off-site generation from wind (as well as conventional/existing power plants using non-renewables that won’t be completely phased out until energy storage technology improves).

There are battery plants and large bourbon distilleries coming  left and right.

The house value would likely never rise much above $40,000, I'm buying this purely for long-term rental income, so per my understanding a conventional refi may never be possible unless I'm misunderstanding something.

If the duplex would cash flow with you in one side, or even break even I'd probably do it now, otherwise you could wait and theoretically put 5% down for your primary and 20% for a pure investment property.

I gift 9V batteries for the smoke detectors and door mats.

Pardon my irony but living your life out of great is never the best strategy.The strategy you mention (and capital gains) in general is pure speculation as the gains might never come and you could even end up with a loss.

Thank you in advanceI think this is purely dependent on your financial situation and your goal.

Their financial state and willingness to accept a such a space is purely due to their age.