If you can make some negative assumptions and it is still cash flowing, you can be more confident that it will work.Also come at it from the other direction and find your break even days.  

My dad built custom homes, was a landlord and flipped a couple houses.

It seems like the assumption in that book is that you can 1) buy a 4-plex for $100k (with a 20% discount at 80k) and b) you can generate $800/mo in cash flow.

In my experience, a reputable company will have zero or very very few bad reviews from customers.

While the foundation might be an issue, its not a pressing issue (my assumption).

The assumption being that when it was constructed it was to code so no change in use to simply split in half.

Start the dishwasher, restock the unit, mop the floors, clean all countertops, bathrooms,  throw away leftover condiments in fridge, vacuum, leave my customized thank you package for the next guest and leave).

I'd stop by the MAAR headquarters; I think they might sell custom signs or if not, they could certainly put you in contact with someone who does.

I had customers showing over $1M buying $150k properties.

Getting familiar with their expertise, how they interact with potential customers and their general availability is useful, however.