To give some context, the house has rotting wood siding that’s about 40 years old.

Again, for an investment, I want to be sure i'm not walking into a money pit such as bad foundations, significant wood rot, a really failed septic, etc.Are you asking about a fixer-upper?

You will need 20-25% down on any commercial property, @Brad Wood.

We used to have a house that had linoleum tiles on top of wood in the kitchen, and whenever we mopped the floor, some of that water would get between the cracks and push some gunk back out.

If your focus is making a beautiful house, you might lose sight of your profits.

Some of them have interesting strategies for finding "diamonds in the rough" hiding in plain sight on the MLS. 

One important factor will be how "out of sight / out of mind" the ADU's location will be.

We repainted the whole unit, everything was honey oak wood-wise, so we stained the doors, updated the hardware, stained the windows, and painted the fireplace and trim white.

People are buying sight unseen.

After reading David Greene's book on long distance real estate investing (twice) and talking to Dave a bit I went for it and things have been going good (knock on wood).